Merge release-20200522.0-88-g527d08f6 (automated)

9 files changed, 138 insertions, 102 deletions

diff --git a/pkg/sentry/platform/kvm/address_space.go b/pkg/sentry/platform/kvm/address_space.go
index be213bfe8..faf1d5e1c 100644
--- a/pkg/sentry/platform/kvm/address_space.go
+++ b/pkg/sentry/platform/kvm/address_space.go
@@ -26,16 +26,15 @@ import (
 
 // dirtySet tracks vCPUs for invalidation.
 type dirtySet struct {
-	vCPUs []uint64
+	vCPUMasks []uint64
 }
 
 // forEach iterates over all CPUs in the dirty set.
+//
+//go:nosplit
 func (ds *dirtySet) forEach(m *machine, fn func(c *vCPU)) {
-	m.mu.RLock()
-	defer m.mu.RUnlock()
-
-	for index := range ds.vCPUs {
-		mask := atomic.SwapUint64(&ds.vCPUs[index], 0)
+	for index := range ds.vCPUMasks {
+		mask := atomic.SwapUint64(&ds.vCPUMasks[index], 0)
 		if mask != 0 {
 			for bit := 0; bit < 64; bit++ {
 				if mask&(1<<uint64(bit)) == 0 {
@@ -54,7 +53,7 @@ func (ds *dirtySet) mark(c *vCPU) bool {
 	index := uint64(c.id) / 64
 	bit := uint64(1) << uint(c.id%64)
 
-	oldValue := atomic.LoadUint64(&ds.vCPUs[index])
+	oldValue := atomic.LoadUint64(&ds.vCPUMasks[index])
 	if oldValue&bit != 0 {
 		return false // Not clean.
 	}
@@ -62,7 +61,7 @@ func (ds *dirtySet) mark(c *vCPU) bool {
 	// Set the bit unilaterally, and ensure that a flush takes place. Note
 	// that it's possible for races to occur here, but since the flush is
 	// taking place long after these lines there's no race in practice.
-	atomicbitops.OrUint64(&ds.vCPUs[index], bit)
+	atomicbitops.OrUint64(&ds.vCPUMasks[index], bit)
 	return true // Previously clean.
 }
 
@@ -113,7 +112,12 @@ type hostMapEntry struct {
 	length uintptr
 }
 
-func (as *addressSpace) mapHost(addr usermem.Addr, m hostMapEntry, at usermem.AccessType) (inv bool) {
+// mapLocked maps the given host entry.
+//
+// +checkescape:hard,stack
+//
+//go:nosplit
+func (as *addressSpace) mapLocked(addr usermem.Addr, m hostMapEntry, at usermem.AccessType) (inv bool) {
 	for m.length > 0 {
 		physical, length, ok := translateToPhysical(m.addr)
 		if !ok {
@@ -133,18 +137,10 @@ func (as *addressSpace) mapHost(addr usermem.Addr, m hostMapEntry, at usermem.Ac
 		// important; if the pagetable mappings were installed before
 		// ensuring the physical pages were available, then some other
 		// thread could theoretically access them.
-		//
-		// Due to the way KVM's shadow paging implementation works,
-		// modifications to the page tables while in host mode may not
-		// be trapped, leading to the shadow pages being out of sync.
-		// Therefore, we need to ensure that we are in guest mode for
-		// page table modifications. See the call to bluepill, below.
-		as.machine.retryInGuest(func() {
-			inv = as.pageTables.Map(addr, length, pagetables.MapOpts{
-				AccessType: at,
-				User:       true,
-			}, physical) || inv
-		})
+		inv = as.pageTables.Map(addr, length, pagetables.MapOpts{
+			AccessType: at,
+			User:       true,
+		}, physical) || inv
 		m.addr += length
 		m.length -= length
 		addr += usermem.Addr(length)
@@ -176,6 +172,10 @@ func (as *addressSpace) MapFile(addr usermem.Addr, f platform.File, fr platform.
 		return err
 	}
 
+	// See block in mapLocked.
+	as.pageTables.Allocator.(*allocator).cpu = as.machine.Get()
+	defer as.machine.Put(as.pageTables.Allocator.(*allocator).cpu)
+
 	// Map the mappings in the sentry's address space (guest physical memory)
 	// into the application's address space (guest virtual memory).
 	inv := false
@@ -190,7 +190,12 @@ func (as *addressSpace) MapFile(addr usermem.Addr, f platform.File, fr platform.
 				_ = s[i] // Touch to commit.
 			}
 		}
-		prev := as.mapHost(addr, hostMapEntry{
+
+		// See bluepill_allocator.go.
+		bluepill(as.pageTables.Allocator.(*allocator).cpu)
+
+		// Perform the mapping.
+		prev := as.mapLocked(addr, hostMapEntry{
 			addr:   b.Addr(),
 			length: uintptr(b.Len()),
 		}, at)
@@ -204,17 +209,27 @@ func (as *addressSpace) MapFile(addr usermem.Addr, f platform.File, fr platform.
 	return nil
 }
 
+// unmapLocked is an escape-checked wrapped around Unmap.
+//
+// +checkescape:hard,stack
+//
+//go:nosplit
+func (as *addressSpace) unmapLocked(addr usermem.Addr, length uint64) bool {
+	return as.pageTables.Unmap(addr, uintptr(length))
+}
+
 // Unmap unmaps the given range by calling pagetables.PageTables.Unmap.
 func (as *addressSpace) Unmap(addr usermem.Addr, length uint64) {
 	as.mu.Lock()
 	defer as.mu.Unlock()
 
-	// See above re: retryInGuest.
-	var prev bool
-	as.machine.retryInGuest(func() {
-		prev = as.pageTables.Unmap(addr, uintptr(length)) || prev
-	})
-	if prev {
+	// See above & bluepill_allocator.go.
+	as.pageTables.Allocator.(*allocator).cpu = as.machine.Get()
+	defer as.machine.Put(as.pageTables.Allocator.(*allocator).cpu)
+	bluepill(as.pageTables.Allocator.(*allocator).cpu)
+
+	if prev := as.unmapLocked(addr, length); prev {
+		// Invalidate all active vCPUs.
 		as.invalidate()
 
 		// Recycle any freed intermediate pages.
@@ -227,7 +242,7 @@ func (as *addressSpace) Release() {
 	as.Unmap(0, ^uint64(0))
 
 	// Free all pages from the allocator.
-	as.pageTables.Allocator.(allocator).base.Drain()
+	as.pageTables.Allocator.(*allocator).base.Drain()
 
 	// Drop all cached machine references.
 	as.machine.dropPageTables(as.pageTables)
diff --git a/pkg/sentry/platform/kvm/allocator.go b/pkg/sentry/platform/kvm/bluepill_allocator.go
index 3f35414bb..9485e1301 100644
--- a/pkg/sentry/platform/kvm/allocator.go
+++ b/pkg/sentry/platform/kvm/bluepill_allocator.go
@@ -21,56 +21,80 @@ import (
 )
 
 type allocator struct {
-	base *pagetables.RuntimeAllocator
+	base pagetables.RuntimeAllocator
+
+	// cpu must be set prior to any pagetable operation.
+	//
+	// Due to the way KVM's shadow paging implementation works,
+	// modifications to the page tables while in host mode may not be
+	// trapped, leading to the shadow pages being out of sync.  Therefore,
+	// we need to ensure that we are in guest mode for page table
+	// modifications. See the call to bluepill, below.
+	cpu *vCPU
 }
 
 // newAllocator is used to define the allocator.
-func newAllocator() allocator {
-	return allocator{
-		base: pagetables.NewRuntimeAllocator(),
-	}
+func newAllocator() *allocator {
+	a := new(allocator)
+	a.base.Init()
+	return a
 }
 
 // NewPTEs implements pagetables.Allocator.NewPTEs.
 //
+// +checkescape:all
+//
 //go:nosplit
-func (a allocator) NewPTEs() *pagetables.PTEs {
-	return a.base.NewPTEs()
+func (a *allocator) NewPTEs() *pagetables.PTEs {
+	ptes := a.base.NewPTEs() // escapes: bluepill below.
+	if a.cpu != nil {
+		bluepill(a.cpu)
+	}
+	return ptes
 }
 
 // PhysicalFor returns the physical address for a set of PTEs.
 //
+// +checkescape:all
+//
 //go:nosplit
-func (a allocator) PhysicalFor(ptes *pagetables.PTEs) uintptr {
+func (a *allocator) PhysicalFor(ptes *pagetables.PTEs) uintptr {
 	virtual := a.base.PhysicalFor(ptes)
 	physical, _, ok := translateToPhysical(virtual)
 	if !ok {
-		panic(fmt.Sprintf("PhysicalFor failed for %p", ptes))
+		panic(fmt.Sprintf("PhysicalFor failed for %p", ptes)) // escapes: panic.
 	}
 	return physical
 }
 
 // LookupPTEs implements pagetables.Allocator.LookupPTEs.
 //
+// +checkescape:all
+//
 //go:nosplit
-func (a allocator) LookupPTEs(physical uintptr) *pagetables.PTEs {
+func (a *allocator) LookupPTEs(physical uintptr) *pagetables.PTEs {
 	virtualStart, physicalStart, _, ok := calculateBluepillFault(physical, physicalRegions)
 	if !ok {
-		panic(fmt.Sprintf("LookupPTEs failed for 0x%x", physical))
+		panic(fmt.Sprintf("LookupPTEs failed for 0x%x", physical)) // escapes: panic.
 	}
 	return a.base.LookupPTEs(virtualStart + (physical - physicalStart))
 }
 
 // FreePTEs implements pagetables.Allocator.FreePTEs.
 //
+// +checkescape:all
+//
 //go:nosplit
-func (a allocator) FreePTEs(ptes *pagetables.PTEs) {
-	a.base.FreePTEs(ptes)
+func (a *allocator) FreePTEs(ptes *pagetables.PTEs) {
+	a.base.FreePTEs(ptes) // escapes: bluepill below.
+	if a.cpu != nil {
+		bluepill(a.cpu)
+	}
 }
 
 // Recycle implements pagetables.Allocator.Recycle.
 //
 //go:nosplit
-func (a allocator) Recycle() {
+func (a *allocator) Recycle() {
 	a.base.Recycle()
 }
diff --git a/pkg/sentry/platform/kvm/bluepill_amd64.go b/pkg/sentry/platform/kvm/bluepill_amd64.go
index 133c2203d..ddc1554d5 100644
--- a/pkg/sentry/platform/kvm/bluepill_amd64.go
+++ b/pkg/sentry/platform/kvm/bluepill_amd64.go
@@ -63,6 +63,8 @@ func bluepillArchEnter(context *arch.SignalContext64) *vCPU {
 
 // KernelSyscall handles kernel syscalls.
 //
+// +checkescape:all
+//
 //go:nosplit
 func (c *vCPU) KernelSyscall() {
 	regs := c.Registers()
@@ -72,13 +74,15 @@ func (c *vCPU) KernelSyscall() {
 	// We only trigger a bluepill entry in the bluepill function, and can
 	// therefore be guaranteed that there is no floating point state to be
 	// loaded on resuming from halt. We only worry about saving on exit.
-	ring0.SaveFloatingPoint((*byte)(c.floatingPointState))
+	ring0.SaveFloatingPoint((*byte)(c.floatingPointState)) // escapes: no.
 	ring0.Halt()
-	ring0.WriteFS(uintptr(regs.Fs_base)) // Reload host segment.
+	ring0.WriteFS(uintptr(regs.Fs_base)) // escapes: no, reload host segment.
 }
 
 // KernelException handles kernel exceptions.
 //
+// +checkescape:all
+//
 //go:nosplit
 func (c *vCPU) KernelException(vector ring0.Vector) {
 	regs := c.Registers()
@@ -89,9 +93,9 @@ func (c *vCPU) KernelException(vector ring0.Vector) {
 		regs.Rip = 0
 	}
 	// See above.
-	ring0.SaveFloatingPoint((*byte)(c.floatingPointState))
+	ring0.SaveFloatingPoint((*byte)(c.floatingPointState)) // escapes: no.
 	ring0.Halt()
-	ring0.WriteFS(uintptr(regs.Fs_base)) // Reload host segment.
+	ring0.WriteFS(uintptr(regs.Fs_base)) // escapes: no; reload host segment.
 }
 
 // bluepillArchExit is called during bluepillEnter.
diff --git a/pkg/sentry/platform/kvm/bluepill_arm64.go b/pkg/sentry/platform/kvm/bluepill_arm64.go
index c215d443c..83643c602 100644
--- a/pkg/sentry/platform/kvm/bluepill_arm64.go
+++ b/pkg/sentry/platform/kvm/bluepill_arm64.go
@@ -66,6 +66,8 @@ func bluepillArchExit(c *vCPU, context *arch.SignalContext64) {
 
 // KernelSyscall handles kernel syscalls.
 //
+// +checkescape:all
+//
 //go:nosplit
 func (c *vCPU) KernelSyscall() {
 	regs := c.Registers()
@@ -88,6 +90,8 @@ func (c *vCPU) KernelSyscall() {
 
 // KernelException handles kernel exceptions.
 //
+// +checkescape:all
+//
 //go:nosplit
 func (c *vCPU) KernelException(vector ring0.Vector) {
 	regs := c.Registers()
diff --git a/pkg/sentry/platform/kvm/bluepill_unsafe.go b/pkg/sentry/platform/kvm/bluepill_unsafe.go
index 2407014e9..c025aa0bb 100644
--- a/pkg/sentry/platform/kvm/bluepill_unsafe.go
+++ b/pkg/sentry/platform/kvm/bluepill_unsafe.go
@@ -64,6 +64,8 @@ func bluepillArchContext(context unsafe.Pointer) *arch.SignalContext64 {
 // signal stack. It should only execute raw system calls and functions that are
 // explicitly marked go:nosplit.
 //
+// +checkescape:all
+//
 //go:nosplit
 func bluepillHandler(context unsafe.Pointer) {
 	// Sanitize the registers; interrupts must always be disabled.
@@ -82,7 +84,8 @@ func bluepillHandler(context unsafe.Pointer) {
 	}
 
 	for {
-		switch _, _, errno := syscall.RawSyscall(syscall.SYS_IOCTL, uintptr(c.fd), _KVM_RUN, 0); errno {
+		_, _, errno := syscall.RawSyscall(syscall.SYS_IOCTL, uintptr(c.fd), _KVM_RUN, 0) // escapes: no.
+		switch errno {
 		case 0: // Expected case.
 		case syscall.EINTR:
 			// First, we process whatever pending signal
@@ -90,7 +93,7 @@ func bluepillHandler(context unsafe.Pointer) {
 			// currently, all signals are masked and the signal
 			// must have been delivered directly to this thread.
 			timeout := syscall.Timespec{}
-			sig, _, errno := syscall.RawSyscall6(
+			sig, _, errno := syscall.RawSyscall6( // escapes: no.
 				syscall.SYS_RT_SIGTIMEDWAIT,
 				uintptr(unsafe.Pointer(&bounceSignalMask)),
 				0,                                 // siginfo.
@@ -125,7 +128,7 @@ func bluepillHandler(context unsafe.Pointer) {
 			// MMIO exit we receive EFAULT from the run ioctl. We
 			// always inject an NMI here since we may be in kernel
 			// mode and have interrupts disabled.
-			if _, _, errno := syscall.RawSyscall(
+			if _, _, errno := syscall.RawSyscall( // escapes: no.
 				syscall.SYS_IOCTL,
 				uintptr(c.fd),
 				_KVM_NMI, 0); errno != 0 {
diff --git a/pkg/sentry/platform/kvm/machine.go b/pkg/sentry/platform/kvm/machine.go
index f1afc74dc..6c54712d1 100644
--- a/pkg/sentry/platform/kvm/machine.go
+++ b/pkg/sentry/platform/kvm/machine.go
@@ -52,16 +52,19 @@ type machine struct {
 	// available is notified when vCPUs are available.
 	available sync.Cond
 
-	// vCPUs are the machine vCPUs.
+	// vCPUsByTID are the machine vCPUs.
 	//
 	// These are populated dynamically.
-	vCPUs map[uint64]*vCPU
+	vCPUsByTID map[uint64]*vCPU
 
 	// vCPUsByID are the machine vCPUs, can be indexed by the vCPU's ID.
-	vCPUsByID map[int]*vCPU
+	vCPUsByID []*vCPU
 
 	// maxVCPUs is the maximum number of vCPUs supported by the machine.
 	maxVCPUs int
+
+	// nextID is the next vCPU ID.
+	nextID uint32
 }
 
 const (
@@ -137,9 +140,8 @@ type dieState struct {
 //
 // Precondition: mu must be held.
 func (m *machine) newVCPU() *vCPU {
-	id := len(m.vCPUs)
-
 	// Create the vCPU.
+	id := int(atomic.AddUint32(&m.nextID, 1) - 1)
 	fd, _, errno := syscall.RawSyscall(syscall.SYS_IOCTL, uintptr(m.fd), _KVM_CREATE_VCPU, uintptr(id))
 	if errno != 0 {
 		panic(fmt.Sprintf("error creating new vCPU: %v", errno))
@@ -176,11 +178,7 @@ func (m *machine) newVCPU() *vCPU {
 // newMachine returns a new VM context.
 func newMachine(vm int) (*machine, error) {
 	// Create the machine.
-	m := &machine{
-		fd:        vm,
-		vCPUs:     make(map[uint64]*vCPU),
-		vCPUsByID: make(map[int]*vCPU),
-	}
+	m := &machine{fd: vm}
 	m.available.L = &m.mu
 	m.kernel.Init(ring0.KernelOpts{
 		PageTables: pagetables.New(newAllocator()),
@@ -194,6 +192,10 @@ func newMachine(vm int) (*machine, error) {
 	}
 	log.Debugf("The maximum number of vCPUs is %d.", m.maxVCPUs)
 
+	// Create the vCPUs map/slices.
+	m.vCPUsByTID = make(map[uint64]*vCPU)
+	m.vCPUsByID = make([]*vCPU, m.maxVCPUs)
+
 	// Apply the physical mappings. Note that these mappings may point to
 	// guest physical addresses that are not actually available. These
 	// physical pages are mapped on demand, see kernel_unsafe.go.
@@ -274,6 +276,8 @@ func newMachine(vm int) (*machine, error) {
 // not available. This attempts to be efficient for calls in the hot path.
 //
 // This panics on error.
+//
+//go:nosplit
 func (m *machine) mapPhysical(physical, length uintptr, phyRegions []physicalRegion, flags uint32) {
 	for end := physical + length; physical < end; {
 		_, physicalStart, length, ok := calculateBluepillFault(physical, phyRegions)
@@ -304,7 +308,11 @@ func (m *machine) Destroy() {
 	runtime.SetFinalizer(m, nil)
 
 	// Destroy vCPUs.
-	for _, c := range m.vCPUs {
+	for _, c := range m.vCPUsByID {
+		if c == nil {
+			continue
+		}
+
 		// Ensure the vCPU is not still running in guest mode. This is
 		// possible iff teardown has been done by other threads, and
 		// somehow a single thread has not executed any system calls.
@@ -337,7 +345,7 @@ func (m *machine) Get() *vCPU {
 	tid := procid.Current()
 
 	// Check for an exact match.
-	if c := m.vCPUs[tid]; c != nil {
+	if c := m.vCPUsByTID[tid]; c != nil {
 		c.lock()
 		m.mu.RUnlock()
 		return c
@@ -356,7 +364,7 @@ func (m *machine) Get() *vCPU {
 	tid = procid.Current()
 
 	// Recheck for an exact match.
-	if c := m.vCPUs[tid]; c != nil {
+	if c := m.vCPUsByTID[tid]; c != nil {
 		c.lock()
 		m.mu.Unlock()
 		return c
@@ -364,10 +372,10 @@ func (m *machine) Get() *vCPU {
 
 	for {
 		// Scan for an available vCPU.
-		for origTID, c := range m.vCPUs {
+		for origTID, c := range m.vCPUsByTID {
 			if atomic.CompareAndSwapUint32(&c.state, vCPUReady, vCPUUser) {
-				delete(m.vCPUs, origTID)
-				m.vCPUs[tid] = c
+				delete(m.vCPUsByTID, origTID)
+				m.vCPUsByTID[tid] = c
 				m.mu.Unlock()
 				c.loadSegments(tid)
 				return c
@@ -375,17 +383,17 @@ func (m *machine) Get() *vCPU {
 		}
 
 		// Create a new vCPU (maybe).
-		if len(m.vCPUs) < m.maxVCPUs {
+		if int(m.nextID) < m.maxVCPUs {
 			c := m.newVCPU()
 			c.lock()
-			m.vCPUs[tid] = c
+			m.vCPUsByTID[tid] = c
 			m.mu.Unlock()
 			c.loadSegments(tid)
 			return c
 		}
 
 		// Scan for something not in user mode.
-		for origTID, c := range m.vCPUs {
+		for origTID, c := range m.vCPUsByTID {
 			if !atomic.CompareAndSwapUint32(&c.state, vCPUGuest, vCPUGuest|vCPUWaiter) {
 				continue
 			}
@@ -403,8 +411,8 @@ func (m *machine) Get() *vCPU {
 			}
 
 			// Steal the vCPU.
-			delete(m.vCPUs, origTID)
-			m.vCPUs[tid] = c
+			delete(m.vCPUsByTID, origTID)
+			m.vCPUsByTID[tid] = c
 			m.mu.Unlock()
 			c.loadSegments(tid)
 			return c
@@ -431,7 +439,7 @@ func (m *machine) Put(c *vCPU) {
 // newDirtySet returns a new dirty set.
 func (m *machine) newDirtySet() *dirtySet {
 	return &dirtySet{
-		vCPUs: make([]uint64, (m.maxVCPUs+63)/64, (m.maxVCPUs+63)/64),
+		vCPUMasks: make([]uint64, (m.maxVCPUs+63)/64, (m.maxVCPUs+63)/64),
 	}
 }
 
diff --git a/pkg/sentry/platform/kvm/machine_amd64.go b/pkg/sentry/platform/kvm/machine_amd64.go
index 923ce3909..acc823ba6 100644
--- a/pkg/sentry/platform/kvm/machine_amd64.go
+++ b/pkg/sentry/platform/kvm/machine_amd64.go
@@ -51,9 +51,10 @@ func (m *machine) initArchState() error {
 		recover()
 		debug.SetPanicOnFault(old)
 	}()
-	m.retryInGuest(func() {
-		ring0.SetCPUIDFaulting(true)
-	})
+	c := m.Get()
+	defer m.Put(c)
+	bluepill(c)
+	ring0.SetCPUIDFaulting(true)
 
 	return nil
 }
@@ -89,8 +90,8 @@ func (m *machine) dropPageTables(pt *pagetables.PageTables) {
 	defer m.mu.Unlock()
 
 	// Clear from all PCIDs.
-	for _, c := range m.vCPUs {
-		if c.PCIDs != nil {
+	for _, c := range m.vCPUsByID {
+		if c != nil && c.PCIDs != nil {
 			c.PCIDs.Drop(pt)
 		}
 	}
@@ -335,29 +336,6 @@ func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts, info *arch.SignalInfo)
 	}
 }
 
-// retryInGuest runs the given function in guest mode.
-//
-// If the function does not complete in guest mode (due to execution of a
-// system call due to a GC stall, for example), then it will be retried. The
-// given function must be idempotent as a result of the retry mechanism.
-func (m *machine) retryInGuest(fn func()) {
-	c := m.Get()
-	defer m.Put(c)
-	for {
-		c.ClearErrorCode() // See below.
-		bluepill(c)        // Force guest mode.
-		fn()               // Execute the given function.
-		_, user := c.ErrorCode()
-		if user {
-			// If user is set, then we haven't bailed back to host
-			// mode via a kernel exception or system call. We
-			// consider the full function to have executed in guest
-			// mode and we can return.
-			break
-		}
-	}
-}
-
 // On x86 platform, the flags for "setMemoryRegion" can always be set as 0.
 // There is no need to return read-only physicalRegions.
 func rdonlyRegionsForSetMem() (phyRegions []physicalRegion) {
diff --git a/pkg/sentry/platform/kvm/machine_amd64_unsafe.go b/pkg/sentry/platform/kvm/machine_amd64_unsafe.go
index 7156c245f..290f035dd 100644
--- a/pkg/sentry/platform/kvm/machine_amd64_unsafe.go
+++ b/pkg/sentry/platform/kvm/machine_amd64_unsafe.go
@@ -154,7 +154,7 @@ func (c *vCPU) setUserRegisters(uregs *userRegs) error {
 //
 //go:nosplit
 func (c *vCPU) getUserRegisters(uregs *userRegs) syscall.Errno {
-	if _, _, errno := syscall.RawSyscall(
+	if _, _, errno := syscall.RawSyscall( // escapes: no.
 		syscall.SYS_IOCTL,
 		uintptr(c.fd),
 		_KVM_GET_REGS,
diff --git a/pkg/sentry/platform/kvm/machine_unsafe.go b/pkg/sentry/platform/kvm/machine_unsafe.go
index de7df4f80..9f86f6a7a 100644
--- a/pkg/sentry/platform/kvm/machine_unsafe.go
+++ b/pkg/sentry/platform/kvm/machine_unsafe.go
@@ -115,7 +115,7 @@ func (a *atomicAddressSpace) get() *addressSpace {
 //
 //go:nosplit
 func (c *vCPU) notify() {
-	_, _, errno := syscall.RawSyscall6(
+	_, _, errno := syscall.RawSyscall6( // escapes: no.
 		syscall.SYS_FUTEX,
 		uintptr(unsafe.Pointer(&c.state)),
 		linux.FUTEX_WAKE|linux.FUTEX_PRIVATE_FLAG,