adding the VM-Exit method for Arm64

 On amd64, it uses 'HLT' to leave the guest.
 Unlike amd64, arm64 can only uses mmio_exit/psci to leave the guest.

 So, I designed the HYPERCALL_VMEXIT to be compatible with amd64/arm64.

 To keep it simple, I used the address of exception table as the
 MMIO base address, so that I can trigger a MMIO-EXIT by forcibly writing this space.
 Then, in host user space, I can calculate this address to find out
 which hypercall.

Signed-off-by: Bin Lu <bin.lu@arm.com>

8 files changed, 81 insertions, 16 deletions

diff --git a/pkg/sentry/platform/kvm/bluepill_amd64_unsafe.go b/pkg/sentry/platform/kvm/bluepill_amd64_unsafe.go
index 99cac665d..0d1e83e6c 100644
--- a/pkg/sentry/platform/kvm/bluepill_amd64_unsafe.go
+++ b/pkg/sentry/platform/kvm/bluepill_amd64_unsafe.go
@@ -53,3 +53,10 @@ func dieArchSetup(c *vCPU, context *arch.SignalContext64, guestRegs *userRegs) {
 	context.Rbx = uint64(uintptr(unsafe.Pointer(c)))
 	context.Rip = uint64(dieTrampolineAddr)
 }
+
+// getHypercallID returns hypercall ID.
+//
+//go:nosplit
+func getHypercallID(addr uintptr) int {
+	return _KVM_HYPERCALL_MAX
+}
diff --git a/pkg/sentry/platform/kvm/bluepill_arm64_unsafe.go b/pkg/sentry/platform/kvm/bluepill_arm64_unsafe.go
index 4ca2b7717..abd36f973 100644
--- a/pkg/sentry/platform/kvm/bluepill_arm64_unsafe.go
+++ b/pkg/sentry/platform/kvm/bluepill_arm64_unsafe.go
@@ -61,3 +61,16 @@ func dieArchSetup(c *vCPU, context *arch.SignalContext64, guestRegs *userRegs) {
 func bluepillArchFpContext(context unsafe.Pointer) *arch.FpsimdContext {
 	return &((*arch.SignalContext64)(context).Fpsimd64)
 }
+
+// getHypercallID returns hypercall ID.
+//
+// On Arm64, the MMIO address should be 64-bit aligned.
+//
+//go:nosplit
+func getHypercallID(addr uintptr) int {
+	if addr < arm64HypercallMMIOBase || addr >= (arm64HypercallMMIOBase+_AARCH64_HYPERCALL_MMIO_SIZE) {
+		return _KVM_HYPERCALL_MAX
+	} else {
+		return int(((addr) - arm64HypercallMMIOBase) >> 3)
+	}
+}
diff --git a/pkg/sentry/platform/kvm/bluepill_unsafe.go b/pkg/sentry/platform/kvm/bluepill_unsafe.go
index 9add7c944..e79097b5d 100644
--- a/pkg/sentry/platform/kvm/bluepill_unsafe.go
+++ b/pkg/sentry/platform/kvm/bluepill_unsafe.go
@@ -58,6 +58,24 @@ func bluepillArchContext(context unsafe.Pointer) *arch.SignalContext64 {
 	return &((*arch.UContext64)(context).MContext)
 }
 
+// bluepillHandleHlt is reponsible for handling VM-Exit.
+//
+//go:nosplit
+func bluepillGuestExit(c *vCPU, context unsafe.Pointer) {
+	// Copy out registers.
+	bluepillArchExit(c, bluepillArchContext(context))
+
+	// Return to the vCPUReady state; notify any waiters.
+	user := atomic.LoadUint32(&c.state) & vCPUUser
+	switch atomic.SwapUint32(&c.state, user) {
+	case user | vCPUGuest: // Expected case.
+	case user | vCPUGuest | vCPUWaiter:
+		c.notify()
+	default:
+		throw("invalid state")
+	}
+}
+
 // bluepillHandler is called from the signal stub.
 //
 // The world may be stopped while this is executing, and it executes on the
@@ -156,25 +174,20 @@ func bluepillHandler(context unsafe.Pointer) {
 			c.die(bluepillArchContext(context), "debug")
 			return
 		case _KVM_EXIT_HLT:
-			// Copy out registers.
-			bluepillArchExit(c, bluepillArchContext(context))
-
-			// Return to the vCPUReady state; notify any waiters.
-			user := atomic.LoadUint32(&c.state) & vCPUUser
-			switch atomic.SwapUint32(&c.state, user) {
-			case user | vCPUGuest: // Expected case.
-			case user | vCPUGuest | vCPUWaiter:
-				c.notify()
-			default:
-				throw("invalid state")
-			}
+			bluepillGuestExit(c, context)
 			return
 		case _KVM_EXIT_MMIO:
+			physical := uintptr(c.runData.data[0])
+			if getHypercallID(physical) == _KVM_HYPERCALL_VMEXIT {
+				bluepillGuestExit(c, context)
+				return
+			}
+
 			// Increment the fault count.
 			atomic.AddUint32(&c.faults, 1)
 
 			// For MMIO, the physical address is the first data item.
-			physical := uintptr(c.runData.data[0])
+			physical = uintptr(c.runData.data[0])
 			virtual, ok := handleBluepillFault(c.machine, physical, physicalRegions, _KVM_MEM_FLAGS_NONE)
 			if !ok {
 				c.die(bluepillArchContext(context), "invalid physical address")
diff --git a/pkg/sentry/platform/kvm/kvm_arm64.go b/pkg/sentry/platform/kvm/kvm_arm64.go
index 29d457a7e..3134a076b 100644
--- a/pkg/sentry/platform/kvm/kvm_arm64.go
+++ b/pkg/sentry/platform/kvm/kvm_arm64.go
@@ -26,6 +26,9 @@ type kvmOneReg struct {
 	addr uint64
 }
 
+// arm64HypercallMMIOBase is MMIO base address used to dispatch hypercalls.
+var arm64HypercallMMIOBase uintptr
+
 const KVM_NR_SPSR = 5
 
 type userFpsimdState struct {
diff --git a/pkg/sentry/platform/kvm/kvm_const.go b/pkg/sentry/platform/kvm/kvm_const.go
index 1d5c77ff4..6a7676468 100644
--- a/pkg/sentry/platform/kvm/kvm_const.go
+++ b/pkg/sentry/platform/kvm/kvm_const.go
@@ -71,3 +71,13 @@ const (
 	_KVM_MEM_READONLY        = uint32(1) << 1
 	_KVM_MEM_FLAGS_NONE      = 0
 )
+
+// KVM hypercall list.
+// Canonical list of hypercalls supported.
+const (
+	// On amd64, it uses 'HLT' to leave the guest.
+	// Unlike amd64, arm64 can only uses mmio_exit/psci to leave the guest.
+	// _KVM_HYPERCALL_VMEXIT is only used on Arm64 for now.
+	_KVM_HYPERCALL_VMEXIT int = iota
+	_KVM_HYPERCALL_MAX
+)
diff --git a/pkg/sentry/platform/kvm/kvm_const_arm64.go b/pkg/sentry/platform/kvm/kvm_const_arm64.go
index 531ae8b1e..6f0539c29 100644
--- a/pkg/sentry/platform/kvm/kvm_const_arm64.go
+++ b/pkg/sentry/platform/kvm/kvm_const_arm64.go
@@ -131,3 +131,10 @@ const (
 	_ESR_SEGV_PEMERR_L2 = 0xe
 	_ESR_SEGV_PEMERR_L3 = 0xf
 )
+
+// Arm64: MMIO base address used to dispatch hypercalls.
+const (
+	// on Arm64, the MMIO address must be 64-bit aligned.
+	// Currently, we only need 1 hypercall: hypercall_vmexit.
+	_AARCH64_HYPERCALL_MMIO_SIZE = 1 << 3
+)
diff --git a/pkg/sentry/platform/kvm/machine_arm64_unsafe.go b/pkg/sentry/platform/kvm/machine_arm64_unsafe.go
index 3c02cef7c..48c834499 100644
--- a/pkg/sentry/platform/kvm/machine_arm64_unsafe.go
+++ b/pkg/sentry/platform/kvm/machine_arm64_unsafe.go
@@ -159,6 +159,10 @@ func (c *vCPU) initArchState() error {
 		return err
 	}
 
+	// Use the address of the exception vector table as
+	// the MMIO address base.
+	arm64HypercallMMIOBase = toLocation
+
 	data = ring0.PsrDefaultSet | ring0.KernelFlagsSet
 	reg.id = _KVM_ARM64_REGS_PSTATE
 	if err := c.setOneRegister(&reg); err != nil {
diff --git a/pkg/sentry/platform/ring0/entry_arm64.s b/pkg/sentry/platform/ring0/entry_arm64.s
index db6465663..2bc5f3ecd 100644
--- a/pkg/sentry/platform/ring0/entry_arm64.s
+++ b/pkg/sentry/platform/ring0/entry_arm64.s
@@ -362,9 +362,17 @@ mmio_exit:
 	MOVD R1, CPU_LAZY_VFP(RSV_REG)
 	VFP_DISABLE
 
-	// MMIO_EXIT.
-	MOVD $0, R9
-	MOVD R0, 0xffff000000001000(R9)
+	// Trigger MMIO_EXIT/_KVM_HYPERCALL_VMEXIT.
+	//
+	// To keep it simple, I used the address of exception table as the
+	// MMIO base address, so that I can trigger a MMIO-EXIT by forcibly writing
+	// a read-only space.
+	// Also, the length is engough to match a sufficient number of hypercall ID.
+	// Then, in host user space, I can calculate this address to find out
+	// which hypercall.
+	MRS VBAR_EL1, R9
+	MOVD R0, 0x0(R9)
+
 	RET
 
 // HaltAndResume halts execution and point the pointer to the resume function.