Merge pull request #5677 from avagin:kvm-mmio

PiperOrigin-RevId: 364728696

pkg/sentry/platform/kvm/machine_amd64.go

@@ -294,6 +294,28 @@ func (c *vCPU) fault(signal int32, info *arch.SignalInfo) (usermem.AccessType, e
 	return accessType, platform.ErrContextSignal
 }
 
+//go:nosplit
+//go:noinline
+func loadByte(ptr *byte) byte {
+	return *ptr
+}
+
+// prefaultFloatingPointState touches each page of the floating point state to
+// be sure that its physical pages are mapped.
+//
+// Otherwise the kernel can trigger KVM_EXIT_MMIO and an instruction that
+// triggered a fault will be emulated by the kvm kernel code, but it can't
+// emulate instructions like xsave and xrstor.
+//
+//go:nosplit
+func prefaultFloatingPointState(data *fpu.State) {
+	size := len(*data)
+	for i := 0; i < size; i += usermem.PageSize {
+		loadByte(&(*data)[i])
+	}
+	loadByte(&(*data)[size-1])
+}
+
 // SwitchToUser unpacks architectural-details.
 func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts, info *arch.SignalInfo) (usermem.AccessType, error) {
 	// Check for canonical addresses.
@@ -324,6 +346,7 @@ func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts, info *arch.SignalInfo)
 	// allocations occur.
 	entersyscall()
 	bluepill(c)
+	prefaultFloatingPointState(switchOpts.FloatingPointState)
 	vector = c.CPU.SwitchToUser(switchOpts)
 	exitsyscall()