Merge release-20190806.1-143-ga8834fc (automated)

17 files changed, 1873 insertions, 80 deletions

diff --git a/pkg/fdchannel/fdchannel_state_autogen.go b/pkg/fdchannel/fdchannel_state_autogen.go
new file mode 100755
index 000000000..8dbf80cba
--- /dev/null
+++ b/pkg/fdchannel/fdchannel_state_autogen.go
@@ -0,0 +1,4 @@
+// automatically generated by stateify.
+
+package fdchannel
+
diff --git a/pkg/fdchannel/fdchannel_unsafe.go b/pkg/fdchannel/fdchannel_unsafe.go
new file mode 100755
index 000000000..367235be5
--- /dev/null
+++ b/pkg/fdchannel/fdchannel_unsafe.go
@@ -0,0 +1,146 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+// Package fdchannel implements passing file descriptors between processes over
+// Unix domain sockets.
+package fdchannel
+
+import (
+	"fmt"
+	"reflect"
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+)
+
+// int32 is the real type of a file descriptor.
+const sizeofInt32 = int(unsafe.Sizeof(int32(0)))
+
+// NewConnectedSockets returns a pair of file descriptors, owned by the caller,
+// representing connected sockets that may be passed to separate calls to
+// NewEndpoint to create connected Endpoints.
+func NewConnectedSockets() ([2]int, error) {
+	return syscall.Socketpair(syscall.AF_UNIX, syscall.SOCK_SEQPACKET|syscall.SOCK_CLOEXEC, 0)
+}
+
+// Endpoint sends file descriptors to, and receives them from, another
+// connected Endpoint.
+//
+// Endpoint is not copyable or movable by value.
+type Endpoint struct {
+	sockfd int32 // accessed using atomic memory operations
+	msghdr syscall.Msghdr
+	cmsg   *syscall.Cmsghdr // followed by sizeofInt32 bytes of data
+}
+
+// Init must be called on zero-value Endpoints before first use. sockfd must be
+// a blocking AF_UNIX SOCK_SEQPACKET socket.
+func (ep *Endpoint) Init(sockfd int) {
+	// "Datagram sockets in various domains (e.g., the UNIX and Internet
+	// domains) permit zero-length datagrams." - recv(2). Experimentally,
+	// sendmsg+recvmsg for a zero-length datagram is slightly faster than
+	// sendmsg+recvmsg for a single byte over a stream socket.
+	cmsgSlice := make([]byte, syscall.CmsgSpace(sizeofInt32))
+	cmsgReflect := (*reflect.SliceHeader)((unsafe.Pointer)(&cmsgSlice))
+	ep.sockfd = int32(sockfd)
+	ep.msghdr.Control = (*byte)((unsafe.Pointer)(cmsgReflect.Data))
+	ep.cmsg = (*syscall.Cmsghdr)((unsafe.Pointer)(cmsgReflect.Data))
+	// ep.msghdr.Controllen and ep.cmsg.* are mutated by recvmsg(2), so they're
+	// set before calling sendmsg/recvmsg.
+}
+
+// NewEndpoint is a convenience function that returns an initialized Endpoint
+// allocated on the heap.
+func NewEndpoint(sockfd int) *Endpoint {
+	ep := &Endpoint{}
+	ep.Init(sockfd)
+	return ep
+}
+
+// Destroy releases resources owned by ep. No other Endpoint methods may be
+// called after Destroy.
+func (ep *Endpoint) Destroy() {
+	// These need not use sync/atomic since there must not be any concurrent
+	// calls to Endpoint methods.
+	if ep.sockfd >= 0 {
+		syscall.Close(int(ep.sockfd))
+		ep.sockfd = -1
+	}
+}
+
+// Shutdown causes concurrent and future calls to ep.SendFD(), ep.RecvFD(), and
+// ep.RecvFDNonblock(), as well as the same calls in the connected Endpoint, to
+// unblock and return errors. It does not wait for concurrent calls to return.
+//
+// Shutdown is the only Endpoint method that may be called concurrently with
+// other methods.
+func (ep *Endpoint) Shutdown() {
+	if sockfd := int(atomic.SwapInt32(&ep.sockfd, -1)); sockfd >= 0 {
+		syscall.Shutdown(sockfd, syscall.SHUT_RDWR)
+		syscall.Close(sockfd)
+	}
+}
+
+// SendFD sends the open file description represented by the given file
+// descriptor to the connected Endpoint.
+func (ep *Endpoint) SendFD(fd int) error {
+	cmsgLen := syscall.CmsgLen(sizeofInt32)
+	ep.cmsg.Level = syscall.SOL_SOCKET
+	ep.cmsg.Type = syscall.SCM_RIGHTS
+	ep.cmsg.SetLen(cmsgLen)
+	*ep.cmsgData() = int32(fd)
+	ep.msghdr.SetControllen(cmsgLen)
+	_, _, e := syscall.Syscall(syscall.SYS_SENDMSG, uintptr(atomic.LoadInt32(&ep.sockfd)), uintptr((unsafe.Pointer)(&ep.msghdr)), 0)
+	if e != 0 {
+		return e
+	}
+	return nil
+}
+
+// RecvFD receives an open file description from the connected Endpoint and
+// returns a file descriptor representing it, owned by the caller.
+func (ep *Endpoint) RecvFD() (int, error) {
+	return ep.recvFD(0)
+}
+
+// RecvFDNonblock receives an open file description from the connected Endpoint
+// and returns a file descriptor representing it, owned by the caller. If there
+// are no pending receivable open file descriptions, RecvFDNonblock returns
+// (<unspecified>, EAGAIN or EWOULDBLOCK).
+func (ep *Endpoint) RecvFDNonblock() (int, error) {
+	return ep.recvFD(syscall.MSG_DONTWAIT)
+}
+
+func (ep *Endpoint) recvFD(flags uintptr) (int, error) {
+	cmsgLen := syscall.CmsgLen(sizeofInt32)
+	ep.msghdr.SetControllen(cmsgLen)
+	_, _, e := syscall.Syscall(syscall.SYS_RECVMSG, uintptr(atomic.LoadInt32(&ep.sockfd)), uintptr((unsafe.Pointer)(&ep.msghdr)), flags|syscall.MSG_TRUNC)
+	if e != 0 {
+		return -1, e
+	}
+	if int(ep.msghdr.Controllen) != cmsgLen {
+		return -1, fmt.Errorf("received control message has incorrect length: got %d, wanted %d", ep.msghdr.Controllen, cmsgLen)
+	}
+	if ep.cmsg.Level != syscall.SOL_SOCKET || ep.cmsg.Type != syscall.SCM_RIGHTS {
+		return -1, fmt.Errorf("received control message has incorrect (level, type): got (%v, %v), wanted (%v, %v)", ep.cmsg.Level, ep.cmsg.Type, syscall.SOL_SOCKET, syscall.SCM_RIGHTS)
+	}
+	return int(*ep.cmsgData()), nil
+}
+
+func (ep *Endpoint) cmsgData() *int32 {
+	// syscall.CmsgLen(0) == syscall.cmsgAlignOf(syscall.SizeofCmsghdr)
+	return (*int32)((unsafe.Pointer)(uintptr((unsafe.Pointer)(ep.cmsg)) + uintptr(syscall.CmsgLen(0))))
+}
diff --git a/pkg/flipcall/ctrl_futex.go b/pkg/flipcall/ctrl_futex.go
new file mode 100755
index 000000000..8390915a2
--- /dev/null
+++ b/pkg/flipcall/ctrl_futex.go
@@ -0,0 +1,176 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package flipcall
+
+import (
+	"encoding/json"
+	"fmt"
+	"math"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/log"
+)
+
+type endpointControlImpl struct {
+	state int32
+}
+
+// Bits in endpointControlImpl.state.
+const (
+	epsBlocked = 1 << iota
+	epsShutdown
+)
+
+func (ep *Endpoint) ctrlInit(opts ...EndpointOption) error {
+	if len(opts) != 0 {
+		return fmt.Errorf("unknown EndpointOption: %T", opts[0])
+	}
+	return nil
+}
+
+type ctrlHandshakeRequest struct{}
+
+type ctrlHandshakeResponse struct{}
+
+func (ep *Endpoint) ctrlConnect() error {
+	if err := ep.enterFutexWait(); err != nil {
+		return err
+	}
+	_, err := ep.futexConnect(&ctrlHandshakeRequest{})
+	ep.exitFutexWait()
+	return err
+}
+
+func (ep *Endpoint) ctrlWaitFirst() error {
+	if err := ep.enterFutexWait(); err != nil {
+		return err
+	}
+	defer ep.exitFutexWait()
+
+	// Wait for the handshake request.
+	if err := ep.futexSwitchFromPeer(); err != nil {
+		return err
+	}
+
+	// Read the handshake request.
+	reqLen := atomic.LoadUint32(ep.dataLen())
+	if reqLen > ep.dataCap {
+		return fmt.Errorf("invalid handshake request length %d (maximum %d)", reqLen, ep.dataCap)
+	}
+	var req ctrlHandshakeRequest
+	if err := json.NewDecoder(ep.NewReader(reqLen)).Decode(&req); err != nil {
+		return fmt.Errorf("error reading handshake request: %v", err)
+	}
+
+	// Write the handshake response.
+	w := ep.NewWriter()
+	if err := json.NewEncoder(w).Encode(ctrlHandshakeResponse{}); err != nil {
+		return fmt.Errorf("error writing handshake response: %v", err)
+	}
+	*ep.dataLen() = w.Len()
+
+	// Return control to the client.
+	raceBecomeInactive()
+	if err := ep.futexSwitchToPeer(); err != nil {
+		return err
+	}
+
+	// Wait for the first non-handshake message.
+	return ep.futexSwitchFromPeer()
+}
+
+func (ep *Endpoint) ctrlRoundTrip() error {
+	if err := ep.futexSwitchToPeer(); err != nil {
+		return err
+	}
+	if err := ep.enterFutexWait(); err != nil {
+		return err
+	}
+	err := ep.futexSwitchFromPeer()
+	ep.exitFutexWait()
+	return err
+}
+
+func (ep *Endpoint) ctrlWakeLast() error {
+	return ep.futexSwitchToPeer()
+}
+
+func (ep *Endpoint) enterFutexWait() error {
+	switch eps := atomic.AddInt32(&ep.ctrl.state, epsBlocked); eps {
+	case epsBlocked:
+		return nil
+	case epsBlocked | epsShutdown:
+		atomic.AddInt32(&ep.ctrl.state, -epsBlocked)
+		return shutdownError{}
+	default:
+		// Most likely due to ep.enterFutexWait() being called concurrently
+		// from multiple goroutines.
+		panic(fmt.Sprintf("invalid flipcall.Endpoint.ctrl.state before flipcall.Endpoint.enterFutexWait(): %v", eps-epsBlocked))
+	}
+}
+
+func (ep *Endpoint) exitFutexWait() {
+	switch eps := atomic.AddInt32(&ep.ctrl.state, -epsBlocked); eps {
+	case 0:
+		return
+	case epsShutdown:
+		// ep.ctrlShutdown() was called while we were blocked, so we are
+		// repsonsible for indicating connection shutdown.
+		ep.shutdownConn()
+	default:
+		panic(fmt.Sprintf("invalid flipcall.Endpoint.ctrl.state after flipcall.Endpoint.exitFutexWait(): %v", eps+epsBlocked))
+	}
+}
+
+func (ep *Endpoint) ctrlShutdown() {
+	// Set epsShutdown to ensure that future calls to ep.enterFutexWait() fail.
+	if atomic.AddInt32(&ep.ctrl.state, epsShutdown)&epsBlocked != 0 {
+		// Wake the blocked thread. This must loop because it's possible that
+		// FUTEX_WAKE occurs after the waiter sets epsBlocked, but before it
+		// blocks in FUTEX_WAIT.
+		for {
+			// Wake MaxInt32 threads to prevent a broken or malicious peer from
+			// swallowing our wakeup by FUTEX_WAITing from multiple threads.
+			if err := ep.futexWakeConnState(math.MaxInt32); err != nil {
+				log.Warningf("failed to FUTEX_WAKE Endpoints: %v", err)
+				break
+			}
+			yieldThread()
+			if atomic.LoadInt32(&ep.ctrl.state)&epsBlocked == 0 {
+				break
+			}
+		}
+	} else {
+		// There is no blocked thread, so we are responsible for indicating
+		// connection shutdown.
+		ep.shutdownConn()
+	}
+}
+
+func (ep *Endpoint) shutdownConn() {
+	switch cs := atomic.SwapUint32(ep.connState(), csShutdown); cs {
+	case ep.activeState:
+		if err := ep.futexWakeConnState(1); err != nil {
+			log.Warningf("failed to FUTEX_WAKE peer Endpoint for shutdown: %v", err)
+		}
+	case ep.inactiveState:
+		// The peer is currently active and will detect shutdown when it tries
+		// to update the connection state.
+	case csShutdown:
+		// The peer also called Endpoint.Shutdown().
+	default:
+		log.Warningf("unexpected connection state before Endpoint.shutdownConn(): %v", cs)
+	}
+}
diff --git a/pkg/flipcall/flipcall.go b/pkg/flipcall/flipcall.go
new file mode 100755
index 000000000..386cee42c
--- /dev/null
+++ b/pkg/flipcall/flipcall.go
@@ -0,0 +1,255 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package flipcall implements a protocol providing Fast Local Interprocess
+// Procedure Calls between mutually-distrusting processes.
+package flipcall
+
+import (
+	"fmt"
+	"math"
+	"sync/atomic"
+	"syscall"
+)
+
+// An Endpoint provides the ability to synchronously transfer data and control
+// to a connected peer Endpoint, which may be in another process.
+//
+// Since the Endpoint control transfer model is synchronous, at any given time
+// one Endpoint "has control" (designated the active Endpoint), and the other
+// is "waiting for control" (designated the inactive Endpoint). Users of the
+// flipcall package designate one Endpoint as the client, which is initially
+// active, and the other as the server, which is initially inactive. See
+// flipcall_example_test.go for usage.
+type Endpoint struct {
+	// packet is a pointer to the beginning of the packet window. (Since this
+	// is a raw OS memory mapping and not a Go object, it does not need to be
+	// represented as an unsafe.Pointer.) packet is immutable.
+	packet uintptr
+
+	// dataCap is the size of the datagram part of the packet window in bytes.
+	// dataCap is immutable.
+	dataCap uint32
+
+	// activeState is csClientActive if this is a client Endpoint and
+	// csServerActive if this is a server Endpoint.
+	activeState uint32
+
+	// inactiveState is csServerActive if this is a client Endpoint and
+	// csClientActive if this is a server Endpoint.
+	inactiveState uint32
+
+	// shutdown is non-zero if Endpoint.Shutdown() has been called, or if the
+	// Endpoint has acknowledged shutdown initiated by the peer. shutdown is
+	// accessed using atomic memory operations.
+	shutdown uint32
+
+	ctrl endpointControlImpl
+}
+
+// EndpointSide indicates which side of a connection an Endpoint belongs to.
+type EndpointSide int
+
+const (
+	// ClientSide indicates that an Endpoint is a client (initially-active;
+	// first method call should be Connect).
+	ClientSide EndpointSide = iota
+
+	// ServerSide indicates that an Endpoint is a server (initially-inactive;
+	// first method call should be RecvFirst.)
+	ServerSide
+)
+
+// Init must be called on zero-value Endpoints before first use. If it
+// succeeds, ep.Destroy() must be called once the Endpoint is no longer in use.
+//
+// pwd represents the packet window used to exchange data with the peer
+// Endpoint. FD may differ between Endpoints if they are in different
+// processes, but must represent the same file. The packet window must
+// initially be filled with zero bytes.
+func (ep *Endpoint) Init(side EndpointSide, pwd PacketWindowDescriptor, opts ...EndpointOption) error {
+	switch side {
+	case ClientSide:
+		ep.activeState = csClientActive
+		ep.inactiveState = csServerActive
+	case ServerSide:
+		ep.activeState = csServerActive
+		ep.inactiveState = csClientActive
+	default:
+		return fmt.Errorf("invalid EndpointSide: %v", side)
+	}
+	if pwd.Length < pageSize {
+		return fmt.Errorf("packet window size (%d) less than minimum (%d)", pwd.Length, pageSize)
+	}
+	if pwd.Length > math.MaxUint32 {
+		return fmt.Errorf("packet window size (%d) exceeds maximum (%d)", pwd.Length, math.MaxUint32)
+	}
+	m, _, e := syscall.RawSyscall6(syscall.SYS_MMAP, 0, uintptr(pwd.Length), syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED, uintptr(pwd.FD), uintptr(pwd.Offset))
+	if e != 0 {
+		return fmt.Errorf("failed to mmap packet window: %v", e)
+	}
+	ep.packet = m
+	ep.dataCap = uint32(pwd.Length) - uint32(PacketHeaderBytes)
+	if err := ep.ctrlInit(opts...); err != nil {
+		ep.unmapPacket()
+		return err
+	}
+	return nil
+}
+
+// NewEndpoint is a convenience function that returns an initialized Endpoint
+// allocated on the heap.
+func NewEndpoint(side EndpointSide, pwd PacketWindowDescriptor, opts ...EndpointOption) (*Endpoint, error) {
+	var ep Endpoint
+	if err := ep.Init(side, pwd, opts...); err != nil {
+		return nil, err
+	}
+	return &ep, nil
+}
+
+// An EndpointOption configures an Endpoint.
+type EndpointOption interface {
+	isEndpointOption()
+}
+
+// Destroy releases resources owned by ep. No other Endpoint methods may be
+// called after Destroy.
+func (ep *Endpoint) Destroy() {
+	ep.unmapPacket()
+}
+
+func (ep *Endpoint) unmapPacket() {
+	syscall.RawSyscall(syscall.SYS_MUNMAP, ep.packet, uintptr(ep.dataCap)+PacketHeaderBytes, 0)
+	ep.packet = 0
+}
+
+// Shutdown causes concurrent and future calls to ep.Connect(), ep.SendRecv(),
+// ep.RecvFirst(), and ep.SendLast(), as well as the same calls in the peer
+// Endpoint, to unblock and return errors. It does not wait for concurrent
+// calls to return. Successive calls to Shutdown have no effect.
+//
+// Shutdown is the only Endpoint method that may be called concurrently with
+// other methods on the same Endpoint.
+func (ep *Endpoint) Shutdown() {
+	if atomic.SwapUint32(&ep.shutdown, 1) != 0 {
+		// ep.Shutdown() has previously been called.
+		return
+	}
+	ep.ctrlShutdown()
+}
+
+// isShutdownLocally returns true if ep.Shutdown() has been called.
+func (ep *Endpoint) isShutdownLocally() bool {
+	return atomic.LoadUint32(&ep.shutdown) != 0
+}
+
+type shutdownError struct{}
+
+// Error implements error.Error.
+func (shutdownError) Error() string {
+	return "flipcall connection shutdown"
+}
+
+// DataCap returns the maximum datagram size supported by ep. Equivalently,
+// DataCap returns len(ep.Data()).
+func (ep *Endpoint) DataCap() uint32 {
+	return ep.dataCap
+}
+
+// Connection state.
+const (
+	// The client is, by definition, initially active, so this must be 0.
+	csClientActive = 0
+	csServerActive = 1
+	csShutdown     = 2
+)
+
+// Connect blocks until the peer Endpoint has called Endpoint.RecvFirst().
+//
+// Preconditions: ep is a client Endpoint. ep.Connect(), ep.RecvFirst(),
+// ep.SendRecv(), and ep.SendLast() have never been called.
+func (ep *Endpoint) Connect() error {
+	err := ep.ctrlConnect()
+	if err == nil {
+		raceBecomeActive()
+	}
+	return err
+}
+
+// RecvFirst blocks until the peer Endpoint calls Endpoint.SendRecv(), then
+// returns the datagram length specified by that call.
+//
+// Preconditions: ep is a server Endpoint. ep.SendRecv(), ep.RecvFirst(), and
+// ep.SendLast() have never been called.
+func (ep *Endpoint) RecvFirst() (uint32, error) {
+	if err := ep.ctrlWaitFirst(); err != nil {
+		return 0, err
+	}
+	raceBecomeActive()
+	recvDataLen := atomic.LoadUint32(ep.dataLen())
+	if recvDataLen > ep.dataCap {
+		return 0, fmt.Errorf("received packet with invalid datagram length %d (maximum %d)", recvDataLen, ep.dataCap)
+	}
+	return recvDataLen, nil
+}
+
+// SendRecv transfers control to the peer Endpoint, causing its call to
+// Endpoint.SendRecv() or Endpoint.RecvFirst() to return with the given
+// datagram length, then blocks until the peer Endpoint calls
+// Endpoint.SendRecv() or Endpoint.SendLast().
+//
+// Preconditions: dataLen <= ep.DataCap(). No previous call to ep.SendRecv() or
+// ep.RecvFirst() has returned an error. ep.SendLast() has never been called.
+// If ep is a client Endpoint, ep.Connect() has previously been called and
+// returned nil.
+func (ep *Endpoint) SendRecv(dataLen uint32) (uint32, error) {
+	if dataLen > ep.dataCap {
+		panic(fmt.Sprintf("attempting to send packet with datagram length %d (maximum %d)", dataLen, ep.dataCap))
+	}
+	// This store can safely be non-atomic: Under correct operation we should
+	// be the only thread writing ep.dataLen(), and ep.ctrlRoundTrip() will
+	// synchronize with the receiver. We will not read from ep.dataLen() until
+	// after ep.ctrlRoundTrip(), so if the peer is mutating it concurrently then
+	// they can only shoot themselves in the foot.
+	*ep.dataLen() = dataLen
+	raceBecomeInactive()
+	if err := ep.ctrlRoundTrip(); err != nil {
+		return 0, err
+	}
+	raceBecomeActive()
+	recvDataLen := atomic.LoadUint32(ep.dataLen())
+	if recvDataLen > ep.dataCap {
+		return 0, fmt.Errorf("received packet with invalid datagram length %d (maximum %d)", recvDataLen, ep.dataCap)
+	}
+	return recvDataLen, nil
+}
+
+// SendLast causes the peer Endpoint's call to Endpoint.SendRecv() or
+// Endpoint.RecvFirst() to return with the given datagram length.
+//
+// Preconditions: dataLen <= ep.DataCap(). No previous call to ep.SendRecv() or
+// ep.RecvFirst() has returned an error. ep.SendLast() has never been called.
+// If ep is a client Endpoint, ep.Connect() has previously been called and
+// returned nil.
+func (ep *Endpoint) SendLast(dataLen uint32) error {
+	if dataLen > ep.dataCap {
+		panic(fmt.Sprintf("attempting to send packet with datagram length %d (maximum %d)", dataLen, ep.dataCap))
+	}
+	*ep.dataLen() = dataLen
+	raceBecomeInactive()
+	if err := ep.ctrlWakeLast(); err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/pkg/flipcall/flipcall_state_autogen.go b/pkg/flipcall/flipcall_state_autogen.go
new file mode 100755
index 000000000..e9371b536
--- /dev/null
+++ b/pkg/flipcall/flipcall_state_autogen.go
@@ -0,0 +1,4 @@
+// automatically generated by stateify.
+
+package flipcall
+
diff --git a/pkg/flipcall/flipcall_unsafe.go b/pkg/flipcall/flipcall_unsafe.go
new file mode 100755
index 000000000..a37952637
--- /dev/null
+++ b/pkg/flipcall/flipcall_unsafe.go
@@ -0,0 +1,87 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package flipcall
+
+import (
+	"reflect"
+	"unsafe"
+
+	"gvisor.dev/gvisor/third_party/gvsync"
+)
+
+// Packets consist of a 16-byte header followed by an arbitrarily-sized
+// datagram. The header consists of:
+//
+// - A 4-byte native-endian connection state.
+//
+// - A 4-byte native-endian datagram length in bytes.
+//
+// - 8 reserved bytes.
+const (
+	// PacketHeaderBytes is the size of a flipcall packet header in bytes. The
+	// maximum datagram size supported by a flipcall connection is equal to the
+	// length of the packet window minus PacketHeaderBytes.
+	//
+	// PacketHeaderBytes is exported to support its use in constant
+	// expressions. Non-constant expressions may prefer to use
+	// PacketWindowLengthForDataCap().
+	PacketHeaderBytes = 16
+)
+
+func (ep *Endpoint) connState() *uint32 {
+	return (*uint32)((unsafe.Pointer)(ep.packet))
+}
+
+func (ep *Endpoint) dataLen() *uint32 {
+	return (*uint32)((unsafe.Pointer)(ep.packet + 4))
+}
+
+// Data returns the datagram part of ep's packet window as a byte slice.
+//
+// Note that the packet window is shared with the potentially-untrusted peer
+// Endpoint, which may concurrently mutate the contents of the packet window.
+// Thus:
+//
+// - Readers must not assume that two reads of the same byte in Data() will
+// return the same result. In other words, readers should read any given byte
+// in Data() at most once.
+//
+// - Writers must not assume that they will read back the same data that they
+// have written. In other words, writers should avoid reading from Data() at
+// all.
+func (ep *Endpoint) Data() []byte {
+	var bs []byte
+	bsReflect := (*reflect.SliceHeader)((unsafe.Pointer)(&bs))
+	bsReflect.Data = ep.packet + PacketHeaderBytes
+	bsReflect.Len = int(ep.dataCap)
+	bsReflect.Cap = int(ep.dataCap)
+	return bs
+}
+
+// ioSync is a dummy variable used to indicate synchronization to the Go race
+// detector. Compare syscall.ioSync.
+var ioSync int64
+
+func raceBecomeActive() {
+	if gvsync.RaceEnabled {
+		gvsync.RaceAcquire((unsafe.Pointer)(&ioSync))
+	}
+}
+
+func raceBecomeInactive() {
+	if gvsync.RaceEnabled {
+		gvsync.RaceReleaseMerge((unsafe.Pointer)(&ioSync))
+	}
+}
diff --git a/pkg/flipcall/futex_linux.go b/pkg/flipcall/futex_linux.go
new file mode 100755
index 000000000..b127a2bbb
--- /dev/null
+++ b/pkg/flipcall/futex_linux.go
@@ -0,0 +1,118 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build linux
+
+package flipcall
+
+import (
+	"encoding/json"
+	"fmt"
+	"runtime"
+	"sync/atomic"
+	"syscall"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+)
+
+func (ep *Endpoint) futexConnect(req *ctrlHandshakeRequest) (ctrlHandshakeResponse, error) {
+	var resp ctrlHandshakeResponse
+
+	// Write the handshake request.
+	w := ep.NewWriter()
+	if err := json.NewEncoder(w).Encode(req); err != nil {
+		return resp, fmt.Errorf("error writing handshake request: %v", err)
+	}
+	*ep.dataLen() = w.Len()
+
+	// Exchange control with the server.
+	if err := ep.futexSwitchToPeer(); err != nil {
+		return resp, err
+	}
+	if err := ep.futexSwitchFromPeer(); err != nil {
+		return resp, err
+	}
+
+	// Read the handshake response.
+	respLen := atomic.LoadUint32(ep.dataLen())
+	if respLen > ep.dataCap {
+		return resp, fmt.Errorf("invalid handshake response length %d (maximum %d)", respLen, ep.dataCap)
+	}
+	if err := json.NewDecoder(ep.NewReader(respLen)).Decode(&resp); err != nil {
+		return resp, fmt.Errorf("error reading handshake response: %v", err)
+	}
+
+	return resp, nil
+}
+
+func (ep *Endpoint) futexSwitchToPeer() error {
+	// Update connection state to indicate that the peer should be active.
+	if !atomic.CompareAndSwapUint32(ep.connState(), ep.activeState, ep.inactiveState) {
+		switch cs := atomic.LoadUint32(ep.connState()); cs {
+		case csShutdown:
+			return shutdownError{}
+		default:
+			return fmt.Errorf("unexpected connection state before FUTEX_WAKE: %v", cs)
+		}
+	}
+
+	// Wake the peer's Endpoint.futexSwitchFromPeer().
+	if err := ep.futexWakeConnState(1); err != nil {
+		return fmt.Errorf("failed to FUTEX_WAKE peer Endpoint: %v", err)
+	}
+	return nil
+}
+
+func (ep *Endpoint) futexSwitchFromPeer() error {
+	for {
+		switch cs := atomic.LoadUint32(ep.connState()); cs {
+		case ep.activeState:
+			return nil
+		case ep.inactiveState:
+			if ep.isShutdownLocally() {
+				return shutdownError{}
+			}
+			if err := ep.futexWaitConnState(ep.inactiveState); err != nil {
+				return fmt.Errorf("failed to FUTEX_WAIT for peer Endpoint: %v", err)
+			}
+			continue
+		case csShutdown:
+			return shutdownError{}
+		default:
+			return fmt.Errorf("unexpected connection state before FUTEX_WAIT: %v", cs)
+		}
+	}
+}
+
+func (ep *Endpoint) futexWakeConnState(numThreads int32) error {
+	if _, _, e := syscall.RawSyscall(syscall.SYS_FUTEX, ep.packet, linux.FUTEX_WAKE, uintptr(numThreads)); e != 0 {
+		return e
+	}
+	return nil
+}
+
+func (ep *Endpoint) futexWaitConnState(curState uint32) error {
+	_, _, e := syscall.Syscall6(syscall.SYS_FUTEX, ep.packet, linux.FUTEX_WAIT, uintptr(curState), 0, 0, 0)
+	if e != 0 && e != syscall.EAGAIN && e != syscall.EINTR {
+		return e
+	}
+	return nil
+}
+
+func yieldThread() {
+	syscall.Syscall(syscall.SYS_SCHED_YIELD, 0, 0, 0)
+	// The thread we're trying to yield to may be waiting for a Go runtime P.
+	// runtime.Gosched() will hand off ours if necessary.
+	runtime.Gosched()
+}
diff --git a/pkg/flipcall/io.go b/pkg/flipcall/io.go
new file mode 100755
index 000000000..85e40b932
--- /dev/null
+++ b/pkg/flipcall/io.go
@@ -0,0 +1,113 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package flipcall
+
+import (
+	"fmt"
+	"io"
+)
+
+// DatagramReader implements io.Reader by reading a datagram from an Endpoint's
+// packet window. Its use is optional; users that can use Endpoint.Data() more
+// efficiently are advised to do so.
+type DatagramReader struct {
+	ep  *Endpoint
+	off uint32
+	end uint32
+}
+
+// Init must be called on zero-value DatagramReaders before first use.
+//
+// Preconditions: dataLen is 0, or was returned by a previous call to
+// ep.RecvFirst() or ep.SendRecv().
+func (r *DatagramReader) Init(ep *Endpoint, dataLen uint32) {
+	r.ep = ep
+	r.Reset(dataLen)
+}
+
+// Reset causes r to begin reading a new datagram of the given length from the
+// associated Endpoint.
+//
+// Preconditions: dataLen is 0, or was returned by a previous call to the
+// associated Endpoint's RecvFirst() or SendRecv() methods.
+func (r *DatagramReader) Reset(dataLen uint32) {
+	if dataLen > r.ep.dataCap {
+		panic(fmt.Sprintf("invalid dataLen (%d) > ep.dataCap (%d)", dataLen, r.ep.dataCap))
+	}
+	r.off = 0
+	r.end = dataLen
+}
+
+// NewReader is a convenience function that returns an initialized
+// DatagramReader allocated on the heap.
+//
+// Preconditions: dataLen was returned by a previous call to ep.RecvFirst() or
+// ep.SendRecv().
+func (ep *Endpoint) NewReader(dataLen uint32) *DatagramReader {
+	r := &DatagramReader{}
+	r.Init(ep, dataLen)
+	return r
+}
+
+// Read implements io.Reader.Read.
+func (r *DatagramReader) Read(dst []byte) (int, error) {
+	n := copy(dst, r.ep.Data()[r.off:r.end])
+	r.off += uint32(n)
+	if r.off == r.end {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+// DatagramWriter implements io.Writer by writing a datagram to an Endpoint's
+// packet window. Its use is optional; users that can use Endpoint.Data() more
+// efficiently are advised to do so.
+type DatagramWriter struct {
+	ep  *Endpoint
+	off uint32
+}
+
+// Init must be called on zero-value DatagramWriters before first use.
+func (w *DatagramWriter) Init(ep *Endpoint) {
+	w.ep = ep
+}
+
+// Reset causes w to begin writing a new datagram to the associated Endpoint.
+func (w *DatagramWriter) Reset() {
+	w.off = 0
+}
+
+// NewWriter is a convenience function that returns an initialized
+// DatagramWriter allocated on the heap.
+func (ep *Endpoint) NewWriter() *DatagramWriter {
+	w := &DatagramWriter{}
+	w.Init(ep)
+	return w
+}
+
+// Write implements io.Writer.Write.
+func (w *DatagramWriter) Write(src []byte) (int, error) {
+	n := copy(w.ep.Data()[w.off:w.ep.dataCap], src)
+	w.off += uint32(n)
+	if n != len(src) {
+		return n, fmt.Errorf("datagram would exceed maximum size of %d bytes", w.ep.dataCap)
+	}
+	return n, nil
+}
+
+// Len returns the length of the written datagram.
+func (w *DatagramWriter) Len() uint32 {
+	return w.off
+}
diff --git a/pkg/flipcall/packet_window_allocator.go b/pkg/flipcall/packet_window_allocator.go
new file mode 100755
index 000000000..ccb918fab
--- /dev/null
+++ b/pkg/flipcall/packet_window_allocator.go
@@ -0,0 +1,166 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package flipcall
+
+import (
+	"fmt"
+	"math/bits"
+	"os"
+	"syscall"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/memutil"
+)
+
+var (
+	pageSize = os.Getpagesize()
+	pageMask = pageSize - 1
+)
+
+func init() {
+	if bits.OnesCount(uint(pageSize)) != 1 {
+		// This is depended on by roundUpToPage().
+		panic(fmt.Sprintf("system page size (%d) is not a power of 2", pageSize))
+	}
+	if uintptr(pageSize) < PacketHeaderBytes {
+		// This is required since Endpoint.Init() imposes a minimum packet
+		// window size of 1 page.
+		panic(fmt.Sprintf("system page size (%d) is less than packet header size (%d)", pageSize, PacketHeaderBytes))
+	}
+}
+
+// PacketWindowDescriptor represents a packet window, a range of pages in a
+// shared memory file that is used to exchange packets between partner
+// Endpoints.
+type PacketWindowDescriptor struct {
+	// FD is the file descriptor representing the shared memory file.
+	FD int
+
+	// Offset is the offset into the shared memory file at which the packet
+	// window begins.
+	Offset int64
+
+	// Length is the size of the packet window in bytes.
+	Length int
+}
+
+// PacketWindowLengthForDataCap returns the minimum packet window size required
+// to accommodate datagrams of the given size in bytes.
+func PacketWindowLengthForDataCap(dataCap uint32) int {
+	return roundUpToPage(int(dataCap) + int(PacketHeaderBytes))
+}
+
+func roundUpToPage(x int) int {
+	return (x + pageMask) &^ pageMask
+}
+
+// A PacketWindowAllocator owns a shared memory file, and allocates packet
+// windows from it.
+type PacketWindowAllocator struct {
+	fd        int
+	nextAlloc int64
+	fileSize  int64
+}
+
+// Init must be called on zero-value PacketWindowAllocators before first use.
+// If it succeeds, Destroy() must be called once the PacketWindowAllocator is
+// no longer in use.
+func (pwa *PacketWindowAllocator) Init() error {
+	fd, err := memutil.CreateMemFD("flipcall_packet_windows", linux.MFD_CLOEXEC|linux.MFD_ALLOW_SEALING)
+	if err != nil {
+		return fmt.Errorf("failed to create memfd: %v", err)
+	}
+	// Apply F_SEAL_SHRINK to prevent either party from causing SIGBUS in the
+	// other by truncating the file, and F_SEAL_SEAL to prevent either party
+	// from applying F_SEAL_GROW or F_SEAL_WRITE.
+	if _, _, e := syscall.RawSyscall(syscall.SYS_FCNTL, uintptr(fd), linux.F_ADD_SEALS, linux.F_SEAL_SHRINK|linux.F_SEAL_SEAL); e != 0 {
+		syscall.Close(fd)
+		return fmt.Errorf("failed to apply memfd seals: %v", e)
+	}
+	pwa.fd = fd
+	return nil
+}
+
+// NewPacketWindowAllocator is a convenience function that returns an
+// initialized PacketWindowAllocator allocated on the heap.
+func NewPacketWindowAllocator() (*PacketWindowAllocator, error) {
+	var pwa PacketWindowAllocator
+	if err := pwa.Init(); err != nil {
+		return nil, err
+	}
+	return &pwa, nil
+}
+
+// Destroy releases resources owned by pwa. This invalidates file descriptors
+// previously returned by pwa.FD() and pwd.Allocate().
+func (pwa *PacketWindowAllocator) Destroy() {
+	syscall.Close(pwa.fd)
+}
+
+// FD represents the file descriptor of the shared memory file backing pwa.
+func (pwa *PacketWindowAllocator) FD() int {
+	return pwa.fd
+}
+
+// Allocate allocates a new packet window of at least the given size and
+// returns a PacketWindowDescriptor representing it.
+//
+// Preconditions: size > 0.
+func (pwa *PacketWindowAllocator) Allocate(size int) (PacketWindowDescriptor, error) {
+	if size <= 0 {
+		return PacketWindowDescriptor{}, fmt.Errorf("invalid size: %d", size)
+	}
+	// Page-align size to ensure that pwa.nextAlloc remains page-aligned.
+	size = roundUpToPage(size)
+	if size <= 0 {
+		return PacketWindowDescriptor{}, fmt.Errorf("size %d overflows after rounding up to page size", size)
+	}
+	end := pwa.nextAlloc + int64(size) // overflow checked by ensureFileSize
+	if err := pwa.ensureFileSize(end); err != nil {
+		return PacketWindowDescriptor{}, err
+	}
+	start := pwa.nextAlloc
+	pwa.nextAlloc = end
+	return PacketWindowDescriptor{
+		FD:     pwa.fd,
+		Offset: start,
+		Length: size,
+	}, nil
+}
+
+func (pwa *PacketWindowAllocator) ensureFileSize(min int64) error {
+	if min <= 0 {
+		return fmt.Errorf("file size would overflow")
+	}
+	if pwa.fileSize >= min {
+		return nil
+	}
+	newSize := 2 * pwa.fileSize
+	if newSize == 0 {
+		newSize = int64(pageSize)
+	}
+	for newSize < min {
+		newNewSize := newSize * 2
+		if newNewSize <= 0 {
+			return fmt.Errorf("file size would overflow")
+		}
+		newSize = newNewSize
+	}
+	if err := syscall.Ftruncate(pwa.fd, newSize); err != nil {
+		return fmt.Errorf("ftruncate failed: %v", err)
+	}
+	pwa.fileSize = newSize
+	return nil
+}
diff --git a/pkg/p9/client.go b/pkg/p9/client.go
index 7dc20aeef..123f54e29 100644
--- a/pkg/p9/client.go
+++ b/pkg/p9/client.go
@@ -20,6 +20,8 @@ import (
 	"sync"
 	"syscall"
 
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/flipcall"
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/unet"
 )
@@ -77,6 +79,45 @@ type Client struct {
 	// fidPool is the collection of available fids.
 	fidPool pool
 
+	// messageSize is the maximum total size of a message.
+	messageSize uint32
+
+	// payloadSize is the maximum payload size of a read or write.
+	//
+	// For large reads and writes this means that the read or write is
+	// broken up into buffer-size/payloadSize requests.
+	payloadSize uint32
+
+	// version is the agreed upon version X of 9P2000.L.Google.X.
+	// version 0 implies 9P2000.L.
+	version uint32
+
+	// sendRecv is the transport function.
+	//
+	// This is determined dynamically based on whether or not the server
+	// supports flipcall channels (preferred as it is faster and more
+	// efficient, and does not require tags).
+	sendRecv func(message, message) error
+
+	// -- below corresponds to sendRecvChannel --
+
+	// channelsMu protects channels.
+	channelsMu sync.Mutex
+
+	// channelsWg is a wait group for active clients.
+	channelsWg sync.WaitGroup
+
+	// channels are the set of initialized IPCs channels.
+	channels []*channel
+
+	// inuse is set when the channels are actually in use.
+	//
+	// This is a fixed-size slice, and the entries will be nil when the
+	// corresponding channel is available.
+	inuse []*channel
+
+	// -- below corresponds to sendRecvLegacy --
+
 	// pending is the set of pending messages.
 	pending   map[Tag]*response
 	pendingMu sync.Mutex
@@ -89,19 +130,6 @@ type Client struct {
 	// Whoever writes to this channel is permitted to call recv. When
 	// finished calling recv, this channel should be emptied.
 	recvr chan bool
-
-	// messageSize is the maximum total size of a message.
-	messageSize uint32
-
-	// payloadSize is the maximum payload size of a read or write
-	// request.  For large reads and writes this means that the
-	// read or write is broken up into buffer-size/payloadSize
-	// requests.
-	payloadSize uint32
-
-	// version is the agreed upon version X of 9P2000.L.Google.X.
-	// version 0 implies 9P2000.L.
-	version uint32
 }
 
 // NewClient creates a new client.  It performs a Tversion exchange with
@@ -138,8 +166,15 @@ func NewClient(socket *unet.Socket, messageSize uint32, version string) (*Client
 		return nil, ErrBadVersionString
 	}
 	for {
+		// Always exchange the version using the legacy version of the
+		// protocol. If the protocol supports flipcall, then we switch
+		// our sendRecv function to use that functionality.  Otherwise,
+		// we stick to sendRecvLegacy.
 		rversion := Rversion{}
-		err := c.sendRecv(&Tversion{Version: versionString(requested), MSize: messageSize}, &rversion)
+		err := c.sendRecvLegacy(&Tversion{
+			Version: versionString(requested),
+			MSize:   messageSize,
+		}, &rversion)
 
 		// The server told us to try again with a lower version.
 		if err == syscall.EAGAIN {
@@ -165,9 +200,125 @@ func NewClient(socket *unet.Socket, messageSize uint32, version string) (*Client
 		c.version = version
 		break
 	}
+
+	// Can we switch to use the more advanced channels and create
+	// independent channels for communication? Prefer it if possible.
+	if versionSupportsFlipcall(c.version) {
+		// Attempt to initialize IPC-based communication.
+		for i := 0; i < channelsPerClient; i++ {
+			if err := c.openChannel(i); err != nil {
+				log.Warningf("error opening flipcall channel: %v", err)
+				break // Stop.
+			}
+		}
+		if len(c.channels) >= 1 {
+			// At least one channel created.
+			c.sendRecv = c.sendRecvChannel
+
+			// If we are using channels for communication, then we must poll
+			// for shutdown events on the main socket. If the socket happens
+			// to shutdown, then we will close the channels as well. This is
+			// necessary because channels can hang forever if the server dies
+			// while we're expecting a response.
+			go c.watch(socket) // S/R-SAFE: not relevant.
+		} else {
+			// Channel setup failed; fallback.
+			c.sendRecv = c.sendRecvLegacy
+		}
+	} else {
+		// No channels available: use the legacy mechanism.
+		c.sendRecv = c.sendRecvLegacy
+	}
+
 	return c, nil
 }
 
+// watch watches the given socket and calls Close on hang up events.
+//
+// This is intended to be called as a goroutine.
+func (c *Client) watch(socket *unet.Socket) {
+	events := []unix.PollFd{
+		unix.PollFd{
+			Fd:     int32(socket.FD()),
+			Events: unix.POLLHUP | unix.POLLRDHUP,
+		},
+	}
+
+	for {
+		// Wait for a shutdown event.
+		n, err := unix.Ppoll(events, nil, nil)
+		if n == 0 || err == syscall.EAGAIN {
+			continue
+		}
+		break
+	}
+
+	// Close everything down: this will kick all active clients off any
+	// pending requests. Note that Close must be safe to call concurrently,
+	// and multiple times (see Close below).
+	c.Close()
+}
+
+// openChannel attempts to open a client channel.
+//
+// Note that this function returns naked errors which should not be propagated
+// directly to a caller. It is expected that the errors will be logged and a
+// fallback path will be used instead.
+func (c *Client) openChannel(id int) error {
+	var (
+		rchannel0 Rchannel
+		rchannel1 Rchannel
+		res       = new(channel)
+	)
+
+	// Open the data channel.
+	if err := c.sendRecvLegacy(&Tchannel{
+		ID:      uint32(id),
+		Control: 0,
+	}, &rchannel0); err != nil {
+		return fmt.Errorf("error handling Tchannel message: %v", err)
+	}
+	if rchannel0.FilePayload() == nil {
+		return fmt.Errorf("missing file descriptor on primary channel")
+	}
+
+	// We don't need to hold this.
+	defer rchannel0.FilePayload().Close()
+
+	// Open the channel for file descriptors.
+	if err := c.sendRecvLegacy(&Tchannel{
+		ID:      uint32(id),
+		Control: 1,
+	}, &rchannel1); err != nil {
+		return err
+	}
+	if rchannel1.FilePayload() == nil {
+		return fmt.Errorf("missing file descriptor on file descriptor channel")
+	}
+
+	// Construct the endpoints.
+	res.desc = flipcall.PacketWindowDescriptor{
+		FD:     rchannel0.FilePayload().FD(),
+		Offset: int64(rchannel0.Offset),
+		Length: int(rchannel0.Length),
+	}
+	if err := res.data.Init(flipcall.ClientSide, res.desc); err != nil {
+		rchannel1.FilePayload().Close()
+		return err
+	}
+
+	// The fds channel owns the control payload, and it will be closed when
+	// the channel object is closed.
+	res.fds.Init(rchannel1.FilePayload().Release())
+
+	// Save the channel.
+	c.channelsMu.Lock()
+	defer c.channelsMu.Unlock()
+	c.channels = append(c.channels, res)
+	c.inuse = append(c.inuse, nil)
+	return nil
+}
+
 // handleOne handles a single incoming message.
 //
 // This should only be called with the token from recvr. Note that the received
@@ -247,10 +398,10 @@ func (c *Client) waitAndRecv(done chan error) error {
 	}
 }
 
-// sendRecv performs a roundtrip message exchange.
+// sendRecvLegacy performs a roundtrip message exchange.
 //
 // This is called by internal functions.
-func (c *Client) sendRecv(t message, r message) error {
+func (c *Client) sendRecvLegacy(t message, r message) error {
 	tag, ok := c.tagPool.Get()
 	if !ok {
 		return ErrOutOfTags
@@ -296,12 +447,107 @@ func (c *Client) sendRecv(t message, r message) error {
 	return nil
 }
 
+// sendRecvChannel uses channels to send a message.
+func (c *Client) sendRecvChannel(t message, r message) error {
+	c.channelsMu.Lock()
+	if len(c.channels) == 0 {
+		// No channel available.
+		c.channelsMu.Unlock()
+		return c.sendRecvLegacy(t, r)
+	}
+
+	// Find the last used channel.
+	//
+	// Note that we must add one to the wait group while holding the
+	// channel mutex, in order for the Wait operation to be race-free
+	// below. The Wait operation shuts down all in use channels and
+	// waits for them to return, but must do so holding the mutex.
+	idx := len(c.channels) - 1
+	ch := c.channels[idx]
+	c.channels = c.channels[:idx]
+	c.inuse[idx] = ch
+	c.channelsWg.Add(1)
+	c.channelsMu.Unlock()
+
+	// Ensure that it's connected.
+	if !ch.connected {
+		ch.connected = true
+		if err := ch.data.Connect(); err != nil {
+			// The channel is unusable, so don't return it.
+			ch.Close()
+			c.channelsWg.Done()
+			return err
+		}
+	}
+
+	// Send the message.
+	err := ch.sendRecv(c, t, r)
+	if err != nil {
+		// On shutdown, we'll see ENOENT. This is a normal situation, and
+		// we shouldn't generate a spurious warning message in that case.
+		log.Debugf("error calling sendRecvChannel: %v", err)
+	}
+	c.channelsWg.Done()
+
+	// Return the channel.
+	//
+	// Note that we check the channel from the inuse slice here. This
+	// prevents a race where Close is called, which clears inuse, and
+	// means that we will not actually return the closed channel.
+	c.channelsMu.Lock()
+	if c.inuse[idx] != nil {
+		c.channels = append(c.channels, ch)
+		c.inuse[idx] = nil
+	}
+	c.channelsMu.Unlock()
+
+	return err
+}
+
 // Version returns the negotiated 9P2000.L.Google version number.
 func (c *Client) Version() uint32 {
 	return c.version
 }
 
-// Close closes the underlying socket.
+// Close closes the underlying socket and channels.
+//
+// Because Close may be called asynchronously from watch, it must be
+// safe to call concurrently and multiple times.
 func (c *Client) Close() error {
+	c.channelsMu.Lock()
+	defer c.channelsMu.Unlock()
+
+	// Close all inactive channels.
+	for _, ch := range c.channels {
+		ch.Shutdown()
+		ch.Close()
+	}
+	// Close all active channels.
+	for _, ch := range c.inuse {
+		if ch != nil {
+			log.Debugf("shutting down active channel@%p...", ch)
+			ch.Shutdown()
+		}
+	}
+
+	// Wait for active users.
+	c.channelsWg.Wait()
+
+	// Close all previously active channels.
+	for i, ch := range c.inuse {
+		if ch != nil {
+			ch.Close()
+
+			// Clear the inuse entry here so that it will not be returned
+			// to the channel slice, which is cleared below. See the
+			// comment at the end of sendRecvChannel.
+			c.inuse[i] = nil
+		}
+	}
+	c.channels = nil // Prevent use again.
+
+	// Close the main socket. Note that operation is safe to be called
+	// multiple times, unlikely the channel Close operations above, which
+	// we are careful to ensure aren't called twice.
 	return c.socket.Close()
 }
diff --git a/pkg/p9/handlers.go b/pkg/p9/handlers.go
index 999b4f684..ba9a55d6d 100644
--- a/pkg/p9/handlers.go
+++ b/pkg/p9/handlers.go
@@ -305,7 +305,9 @@ func (t *Tlopen) handle(cs *connState) message {
 	ref.opened = true
 	ref.openFlags = t.Flags
 
-	return &Rlopen{QID: qid, IoUnit: ioUnit, File: osFile}
+	rlopen := &Rlopen{QID: qid, IoUnit: ioUnit}
+	rlopen.SetFilePayload(osFile)
+	return rlopen
 }
 
 func (t *Tlcreate) do(cs *connState, uid UID) (*Rlcreate, error) {
@@ -364,7 +366,9 @@ func (t *Tlcreate) do(cs *connState, uid UID) (*Rlcreate, error) {
 	// Replace the FID reference.
 	cs.InsertFID(t.FID, newRef)
 
-	return &Rlcreate{Rlopen: Rlopen{QID: qid, IoUnit: ioUnit, File: osFile}}, nil
+	rlcreate := &Rlcreate{Rlopen: Rlopen{QID: qid, IoUnit: ioUnit}}
+	rlcreate.SetFilePayload(osFile)
+	return rlcreate, nil
 }
 
 // handle implements handler.handle.
@@ -1287,5 +1291,48 @@ func (t *Tlconnect) handle(cs *connState) message {
 		return newErr(err)
 	}
 
-	return &Rlconnect{File: osFile}
+	rlconnect := &Rlconnect{}
+	rlconnect.SetFilePayload(osFile)
+	return rlconnect
+}
+
+// handle implements handler.handle.
+func (t *Tchannel) handle(cs *connState) message {
+	// Ensure that channels are enabled.
+	if err := cs.initializeChannels(); err != nil {
+		return newErr(err)
+	}
+
+	// Lookup the given channel.
+	ch := cs.lookupChannel(t.ID)
+	if ch == nil {
+		return newErr(syscall.ENOSYS)
+	}
+
+	// Return the payload. Note that we need to duplicate the file
+	// descriptor for the channel allocator, because sending is a
+	// destructive operation between sendRecvLegacy (and now the newer
+	// channel send operations). Same goes for the client FD.
+	rchannel := &Rchannel{
+		Offset: uint64(ch.desc.Offset),
+		Length: uint64(ch.desc.Length),
+	}
+	switch t.Control {
+	case 0:
+		// Open the main data channel.
+		mfd, err := syscall.Dup(int(cs.channelAlloc.FD()))
+		if err != nil {
+			return newErr(err)
+		}
+		rchannel.SetFilePayload(fd.New(mfd))
+	case 1:
+		cfd, err := syscall.Dup(ch.client.FD())
+		if err != nil {
+			return newErr(err)
+		}
+		rchannel.SetFilePayload(fd.New(cfd))
+	default:
+		return newErr(syscall.EINVAL)
+	}
+	return rchannel
 }
diff --git a/pkg/p9/messages.go b/pkg/p9/messages.go
index fd9eb1c5d..ffdd7e8c6 100644
--- a/pkg/p9/messages.go
+++ b/pkg/p9/messages.go
@@ -64,6 +64,21 @@ type filer interface {
 	SetFilePayload(*fd.FD)
 }
 
+// filePayload embeds a File object.
+type filePayload struct {
+	File *fd.FD
+}
+
+// FilePayload returns the file payload.
+func (f *filePayload) FilePayload() *fd.FD {
+	return f.File
+}
+
+// SetFilePayload sets the received file.
+func (f *filePayload) SetFilePayload(file *fd.FD) {
+	f.File = file
+}
+
 // Tversion is a version request.
 type Tversion struct {
 	// MSize is the message size to use.
@@ -524,10 +539,7 @@ type Rlopen struct {
 	// IoUnit is the recommended I/O unit.
 	IoUnit uint32
 
-	// File may be attached via the socket.
-	//
-	// This is an extension specific to this package.
-	File *fd.FD
+	filePayload
 }
 
 // Decode implements encoder.Decode.
@@ -547,16 +559,6 @@ func (*Rlopen) Type() MsgType {
 	return MsgRlopen
 }
 
-// FilePayload returns the file payload.
-func (r *Rlopen) FilePayload() *fd.FD {
-	return r.File
-}
-
-// SetFilePayload sets the received file.
-func (r *Rlopen) SetFilePayload(file *fd.FD) {
-	r.File = file
-}
-
 // String implements fmt.Stringer.
 func (r *Rlopen) String() string {
 	return fmt.Sprintf("Rlopen{QID: %s, IoUnit: %d, File: %v}", r.QID, r.IoUnit, r.File)
@@ -2171,8 +2173,7 @@ func (t *Tlconnect) String() string {
 
 // Rlconnect is a connect response.
 type Rlconnect struct {
-	// File is a host socket.
-	File *fd.FD
+	filePayload
 }
 
 // Decode implements encoder.Decode.
@@ -2186,19 +2187,71 @@ func (*Rlconnect) Type() MsgType {
 	return MsgRlconnect
 }
 
-// FilePayload returns the file payload.
-func (r *Rlconnect) FilePayload() *fd.FD {
-	return r.File
+// String implements fmt.Stringer.
+func (r *Rlconnect) String() string {
+	return fmt.Sprintf("Rlconnect{File: %v}", r.File)
 }
 
-// SetFilePayload sets the received file.
-func (r *Rlconnect) SetFilePayload(file *fd.FD) {
-	r.File = file
+// Tchannel creates a new channel.
+type Tchannel struct {
+	// ID is the channel ID.
+	ID uint32
+
+	// Control is 0 if the Rchannel response should provide the flipcall
+	// component of the channel, and 1 if the Rchannel response should
+	// provide the fdchannel component of the channel.
+	Control uint32
+}
+
+// Decode implements encoder.Decode.
+func (t *Tchannel) Decode(b *buffer) {
+	t.ID = b.Read32()
+	t.Control = b.Read32()
+}
+
+// Encode implements encoder.Encode.
+func (t *Tchannel) Encode(b *buffer) {
+	b.Write32(t.ID)
+	b.Write32(t.Control)
+}
+
+// Type implements message.Type.
+func (*Tchannel) Type() MsgType {
+	return MsgTchannel
 }
 
 // String implements fmt.Stringer.
-func (r *Rlconnect) String() string {
-	return fmt.Sprintf("Rlconnect{File: %v}", r.File)
+func (t *Tchannel) String() string {
+	return fmt.Sprintf("Tchannel{ID: %d, Control: %d}", t.ID, t.Control)
+}
+
+// Rchannel is the channel response.
+type Rchannel struct {
+	Offset uint64
+	Length uint64
+	filePayload
+}
+
+// Decode implements encoder.Decode.
+func (r *Rchannel) Decode(b *buffer) {
+	r.Offset = b.Read64()
+	r.Length = b.Read64()
+}
+
+// Encode implements encoder.Encode.
+func (r *Rchannel) Encode(b *buffer) {
+	b.Write64(r.Offset)
+	b.Write64(r.Length)
+}
+
+// Type implements message.Type.
+func (*Rchannel) Type() MsgType {
+	return MsgRchannel
+}
+
+// String implements fmt.Stringer.
+func (r *Rchannel) String() string {
+	return fmt.Sprintf("Rchannel{Offset: %d, Length: %d}", r.Offset, r.Length)
 }
 
 const maxCacheSize = 3
@@ -2356,4 +2409,6 @@ func init() {
 	msgRegistry.register(MsgRlconnect, func() message { return &Rlconnect{} })
 	msgRegistry.register(MsgTallocate, func() message { return &Tallocate{} })
 	msgRegistry.register(MsgRallocate, func() message { return &Rallocate{} })
+	msgRegistry.register(MsgTchannel, func() message { return &Tchannel{} })
+	msgRegistry.register(MsgRchannel, func() message { return &Rchannel{} })
 }
diff --git a/pkg/p9/p9.go b/pkg/p9/p9.go
index e12831dbd..25530adca 100644
--- a/pkg/p9/p9.go
+++ b/pkg/p9/p9.go
@@ -378,6 +378,8 @@ const (
 	MsgRlconnect            = 137
 	MsgTallocate            = 138
 	MsgRallocate            = 139
+	MsgTchannel             = 250
+	MsgRchannel             = 251
 )
 
 // QIDType represents the file type for QIDs.
diff --git a/pkg/p9/server.go b/pkg/p9/server.go
index b294efbb0..69c886a5d 100644
--- a/pkg/p9/server.go
+++ b/pkg/p9/server.go
@@ -21,6 +21,9 @@ import (
 	"sync/atomic"
 	"syscall"
 
+	"gvisor.dev/gvisor/pkg/fd"
+	"gvisor.dev/gvisor/pkg/fdchannel"
+	"gvisor.dev/gvisor/pkg/flipcall"
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/unet"
 )
@@ -45,7 +48,6 @@ type Server struct {
 }
 
 // NewServer returns a new server.
-//
 func NewServer(attacher Attacher) *Server {
 	return &Server{
 		attacher: attacher,
@@ -85,6 +87,8 @@ type connState struct {
 	// version 0 implies 9P2000.L.
 	version uint32
 
+	// -- below relates to the legacy handler --
+
 	// recvOkay indicates that a receive may start.
 	recvOkay chan bool
 
@@ -93,6 +97,20 @@ type connState struct {
 
 	// sendDone is signalled when a send is finished.
 	sendDone chan error
+
+	// -- below relates to the flipcall handler --
+
+	// channelMu protects below.
+	channelMu sync.Mutex
+
+	// channelWg represents active workers.
+	channelWg sync.WaitGroup
+
+	// channelAlloc allocates channel memory.
+	channelAlloc *flipcall.PacketWindowAllocator
+
+	// channels are the set of initialized channels.
+	channels []*channel
 }
 
 // fidRef wraps a node and tracks references.
@@ -386,6 +404,99 @@ func (cs *connState) WaitTag(t Tag) {
 	<-ch
 }
 
+// initializeChannels initializes all channels.
+//
+// This is a no-op if channels are already initialized.
+func (cs *connState) initializeChannels() (err error) {
+	cs.channelMu.Lock()
+	defer cs.channelMu.Unlock()
+
+	// Initialize our channel allocator.
+	if cs.channelAlloc == nil {
+		alloc, err := flipcall.NewPacketWindowAllocator()
+		if err != nil {
+			return err
+		}
+		cs.channelAlloc = alloc
+	}
+
+	// Create all the channels.
+	for len(cs.channels) < channelsPerClient {
+		res := &channel{
+			done: make(chan struct{}),
+		}
+
+		res.desc, err = cs.channelAlloc.Allocate(channelSize)
+		if err != nil {
+			return err
+		}
+		if err := res.data.Init(flipcall.ServerSide, res.desc); err != nil {
+			return err
+		}
+
+		socks, err := fdchannel.NewConnectedSockets()
+		if err != nil {
+			res.data.Destroy() // Cleanup.
+			return err
+		}
+		res.fds.Init(socks[0])
+		res.client = fd.New(socks[1])
+
+		cs.channels = append(cs.channels, res)
+
+		// Start servicing the channel.
+		//
+		// When we call stop, we will close all the channels and these
+		// routines should finish. We need the wait group to ensure
+		// that active handlers are actually finished before cleanup.
+		cs.channelWg.Add(1)
+		go func() { // S/R-SAFE: Server side.
+			defer cs.channelWg.Done()
+			res.service(cs)
+		}()
+	}
+
+	return nil
+}
+
+// lookupChannel looks up the channel with given id.
+//
+// The function returns nil if no such channel is available.
+func (cs *connState) lookupChannel(id uint32) *channel {
+	cs.channelMu.Lock()
+	defer cs.channelMu.Unlock()
+	if id >= uint32(len(cs.channels)) {
+		return nil
+	}
+	return cs.channels[id]
+}
+
+// handle handles a single message.
+func (cs *connState) handle(m message) (r message) {
+	defer func() {
+		if r == nil {
+			// Don't allow a panic to propagate.
+			recover()
+
+			// Include a useful log message.
+			log.Warningf("panic in handler: %s", debug.Stack())
+
+			// Wrap in an EFAULT error; we don't really have a
+			// better way to describe this kind of error. It will
+			// usually manifest as a result of the test framework.
+			r = newErr(syscall.EFAULT)
+		}
+	}()
+	if handler, ok := m.(handler); ok {
+		// Call the message handler.
+		r = handler.handle(cs)
+	} else {
+		// Produce an ENOSYS error.
+		r = newErr(syscall.ENOSYS)
+	}
+	return
+}
+
 // handleRequest handles a single request.
 //
 // The recvDone channel is signaled when recv is done (with a error if
@@ -428,41 +539,20 @@ func (cs *connState) handleRequest() {
 	}
 
 	// Handle the message.
-	var r message // r is the response.
-	defer func() {
-		if r == nil {
-			// Don't allow a panic to propagate.
-			recover()
+	r := cs.handle(m)
 
-			// Include a useful log message.
-			log.Warningf("panic in handler: %s", debug.Stack())
+	// Clear the tag before sending. That's because as soon as this hits
+	// the wire, the client can legally send the same tag.
+	cs.ClearTag(tag)
 
-			// Wrap in an EFAULT error; we don't really have a
-			// better way to describe this kind of error. It will
-			// usually manifest as a result of the test framework.
-			r = newErr(syscall.EFAULT)
-		}
+	// Send back the result.
+	cs.sendMu.Lock()
+	err = send(cs.conn, tag, r)
+	cs.sendMu.Unlock()
+	cs.sendDone <- err
 
-		// Clear the tag before sending. That's because as soon as this
-		// hits the wire, the client can legally send another message
-		// with the same tag.
-		cs.ClearTag(tag)
-
-		// Send back the result.
-		cs.sendMu.Lock()
-		err = send(cs.conn, tag, r)
-		cs.sendMu.Unlock()
-		cs.sendDone <- err
-	}()
-	if handler, ok := m.(handler); ok {
-		// Call the message handler.
-		r = handler.handle(cs)
-	} else {
-		// Produce an ENOSYS error.
-		r = newErr(syscall.ENOSYS)
-	}
+	// Return the message to the cache.
 	msgRegistry.put(m)
-	m = nil // 'm' should not be touched after this point.
 }
 
 func (cs *connState) handleRequests() {
@@ -477,7 +567,27 @@ func (cs *connState) stop() {
 	close(cs.recvDone)
 	close(cs.sendDone)
 
-	for _, fidRef := range cs.fids {
+	// Free the channels.
+	cs.channelMu.Lock()
+	for _, ch := range cs.channels {
+		ch.Shutdown()
+	}
+	cs.channelWg.Wait()
+	for _, ch := range cs.channels {
+		ch.Close()
+	}
+	cs.channels = nil // Clear.
+	cs.channelMu.Unlock()
+
+	// Free the channel memory.
+	if cs.channelAlloc != nil {
+		cs.channelAlloc.Destroy()
+	}
+
+	// Close all remaining fids.
+	for fid, fidRef := range cs.fids {
+		delete(cs.fids, fid)
+
 		// Drop final reference in the FID table. Note this should
 		// always close the file, since we've ensured that there are no
 		// handlers running via the wait for Pending => 0 below.
@@ -510,7 +620,7 @@ func (cs *connState) service() error {
 				for i := 0; i < pending; i++ {
 					<-cs.sendDone
 				}
-				return err
+				return nil
 			}
 
 			// This handler is now pending.
diff --git a/pkg/p9/transport.go b/pkg/p9/transport.go
index 5648df589..6e8b4bbcd 100644
--- a/pkg/p9/transport.go
+++ b/pkg/p9/transport.go
@@ -54,7 +54,10 @@ const (
 	headerLength uint32 = 7
 
 	// maximumLength is the largest possible message.
-	maximumLength uint32 = 4 * 1024 * 1024
+	maximumLength uint32 = 1 << 20
+
+	// DefaultMessageSize is a sensible default.
+	DefaultMessageSize uint32 = 64 << 10
 
 	// initialBufferLength is the initial data buffer we allocate.
 	initialBufferLength uint32 = 64
diff --git a/pkg/p9/transport_flipcall.go b/pkg/p9/transport_flipcall.go
new file mode 100755
index 000000000..aebb54959
--- /dev/null
+++ b/pkg/p9/transport_flipcall.go
@@ -0,0 +1,254 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package p9
+
+import (
+	"runtime"
+	"syscall"
+
+	"gvisor.dev/gvisor/pkg/fd"
+	"gvisor.dev/gvisor/pkg/fdchannel"
+	"gvisor.dev/gvisor/pkg/flipcall"
+	"gvisor.dev/gvisor/pkg/log"
+)
+
+// channelsPerClient is the number of channels to create per client.
+//
+// While the client and server will generally agree on this number, in reality
+// it's completely up to the server. We simply define a minimum of 2, and a
+// maximum of 4, and select the number of available processes as a tie-breaker.
+// Note that we don't want the number of channels to be too large, because each
+// will account for channelSize memory used, which can be large.
+var channelsPerClient = func() int {
+	n := runtime.NumCPU()
+	if n < 2 {
+		return 2
+	}
+	if n > 4 {
+		return 4
+	}
+	return n
+}()
+
+// channelSize is the channel size to create.
+//
+// We simply ensure that this is larger than the largest possible message size,
+// plus the flipcall packet header, plus the two bytes we write below.
+const channelSize = int(2 + flipcall.PacketHeaderBytes + 2 + maximumLength)
+
+// channel is a fast IPC channel.
+//
+// The same object is used by both the server and client implementations. In
+// general, the client will use only the send and recv methods.
+type channel struct {
+	desc flipcall.PacketWindowDescriptor
+	data flipcall.Endpoint
+	fds  fdchannel.Endpoint
+	buf  buffer
+
+	// -- client only --
+	connected bool
+
+	// -- server only --
+	client *fd.FD
+	done   chan struct{}
+}
+
+// reset resets the channel buffer.
+func (ch *channel) reset(sz uint32) {
+	ch.buf.data = ch.data.Data()[:sz]
+}
+
+// service services the channel.
+func (ch *channel) service(cs *connState) error {
+	rsz, err := ch.data.RecvFirst()
+	if err != nil {
+		return err
+	}
+	for rsz > 0 {
+		m, err := ch.recv(nil, rsz)
+		if err != nil {
+			return err
+		}
+		r := cs.handle(m)
+		msgRegistry.put(m)
+		rsz, err = ch.send(r)
+		if err != nil {
+			return err
+		}
+	}
+	return nil // Done.
+}
+
+// Shutdown shuts down the channel.
+//
+// This must be called before Close.
+func (ch *channel) Shutdown() {
+	ch.data.Shutdown()
+}
+
+// Close closes the channel.
+//
+// This must only be called once, and cannot return an error. Note that
+// synchronization for this method is provided at a high-level, depending on
+// whether it is the client or server. This cannot be called while there are
+// active callers in either service or sendRecv.
+//
+// Precondition: the channel should be shutdown.
+func (ch *channel) Close() error {
+	// Close all backing transports.
+	ch.fds.Destroy()
+	ch.data.Destroy()
+	if ch.client != nil {
+		ch.client.Close()
+	}
+	return nil
+}
+
+// send sends the given message.
+//
+// The return value is the size of the received response. Not that in the
+// server case, this is the size of the next request.
+func (ch *channel) send(m message) (uint32, error) {
+	if log.IsLogging(log.Debug) {
+		log.Debugf("send [channel @%p] %s", ch, m.String())
+	}
+
+	// Send any file payload.
+	sentFD := false
+	if filer, ok := m.(filer); ok {
+		if f := filer.FilePayload(); f != nil {
+			if err := ch.fds.SendFD(f.FD()); err != nil {
+				return 0, syscall.EIO // Map everything to EIO.
+			}
+			f.Close()     // Per sendRecvLegacy.
+			sentFD = true // To mark below.
+		}
+	}
+
+	// Encode the message.
+	//
+	// Note that IPC itself encodes the length of messages, so we don't
+	// need to encode a standard 9P header. We write only the message type.
+	ch.reset(0)
+
+	ch.buf.WriteMsgType(m.Type())
+	if sentFD {
+		ch.buf.Write8(1) // Incoming FD.
+	} else {
+		ch.buf.Write8(0) // No incoming FD.
+	}
+	m.Encode(&ch.buf)
+	ssz := uint32(len(ch.buf.data)) // Updated below.
+
+	// Is there a payload?
+	if payloader, ok := m.(payloader); ok {
+		p := payloader.Payload()
+		copy(ch.data.Data()[ssz:], p)
+		ssz += uint32(len(p))
+	}
+
+	// Perform the one-shot communication.
+	n, err := ch.data.SendRecv(ssz)
+	if err != nil {
+		if n > 0 {
+			return n, nil
+		}
+		return 0, syscall.EIO // See above.
+	}
+
+	return n, nil
+}
+
+// recv decodes a message that exists on the channel.
+//
+// If the passed r is non-nil, then the type must match or an error will be
+// generated. If the passed r is nil, then a new message will be created and
+// returned.
+func (ch *channel) recv(r message, rsz uint32) (message, error) {
+	// Decode the response from the inline buffer.
+	ch.reset(rsz)
+	t := ch.buf.ReadMsgType()
+	hasFD := ch.buf.Read8() != 0
+	if t == MsgRlerror {
+		// Change the message type. We check for this special case
+		// after decoding below, and transform into an error.
+		r = &Rlerror{}
+	} else if r == nil {
+		nr, err := msgRegistry.get(0, t)
+		if err != nil {
+			return nil, err
+		}
+		r = nr // New message.
+	} else if t != r.Type() {
+		// Not an error and not the expected response; propagate.
+		return nil, &ErrBadResponse{Got: t, Want: r.Type()}
+	}
+
+	// Is there a payload? Set to the latter portion.
+	if payloader, ok := r.(payloader); ok {
+		fs := payloader.FixedSize()
+		payloader.SetPayload(ch.buf.data[fs:])
+		ch.buf.data = ch.buf.data[:fs]
+	}
+
+	r.Decode(&ch.buf)
+	if ch.buf.isOverrun() {
+		// Nothing valid was available.
+		log.Debugf("recv [got %d bytes, needed more]", rsz)
+		return nil, ErrNoValidMessage
+	}
+
+	// Read any FD result.
+	if hasFD {
+		if rfd, err := ch.fds.RecvFDNonblock(); err == nil {
+			f := fd.New(rfd)
+			if filer, ok := r.(filer); ok {
+				// Set the payload.
+				filer.SetFilePayload(f)
+			} else {
+				// Don't want the FD.
+				f.Close()
+			}
+		} else {
+			// The header bit was set but nothing came in.
+			log.Warningf("expected FD, got err: %v", err)
+		}
+	}
+
+	// Log a message.
+	if log.IsLogging(log.Debug) {
+		log.Debugf("recv [channel @%p] %s", ch, r.String())
+	}
+
+	// Convert errors appropriately; see above.
+	if rlerr, ok := r.(*Rlerror); ok {
+		return nil, syscall.Errno(rlerr.Error)
+	}
+
+	return r, nil
+}
+
+// sendRecv sends the given message over the channel.
+//
+// This is used by the client.
+func (ch *channel) sendRecv(c *Client, m, r message) error {
+	rsz, err := ch.send(m)
+	if err != nil {
+		return err
+	}
+	_, err = ch.recv(r, rsz)
+	return err
+}
diff --git a/pkg/p9/version.go b/pkg/p9/version.go
index c2a2885ae..f1ffdd23a 100644
--- a/pkg/p9/version.go
+++ b/pkg/p9/version.go
@@ -26,7 +26,7 @@ const (
 	//
 	// Clients are expected to start requesting this version number and
 	// to continuously decrement it until a Tversion request succeeds.
-	highestSupportedVersion uint32 = 7
+	highestSupportedVersion uint32 = 8
 
 	// lowestSupportedVersion is the lowest supported version X in a
 	// version string of the format 9P2000.L.Google.X.
@@ -148,3 +148,10 @@ func VersionSupportsMultiUser(v uint32) bool {
 func versionSupportsTallocate(v uint32) bool {
 	return v >= 7
 }
+
+// versionSupportsFlipcall returns true if version v supports IPC channels from
+// the flipcall package. Note that these must be negotiated, but this version
+// string indicates that such a facility exists.
+func versionSupportsFlipcall(v uint32) bool {
+	return v >= 8
+}