passed the syscall test case 'alarm' on Arm64 platform

This issue was caused by 'restart_syscall'.
The value of Register R0 should be stored after finishing sysemu.
So that we can restore the value and restart syscall.

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

pkg/sentry/arch/arch_aarch64.go

@@ -97,6 +97,9 @@ type State struct {
 
 	// FeatureSet is a pointer to the currently active feature set.
 	FeatureSet *cpuid.FeatureSet
+
+	// OrigR0 stores the value of register R0.
+	OrigR0 uint64
 }
 
 // Proto returns a protobuf representation of the system registers in State.
@@ -146,6 +149,7 @@ func (s *State) Fork() State {
 		Regs:           s.Regs,
 		aarch64FPState: s.aarch64FPState.fork(),
 		FeatureSet:     s.FeatureSet,
+		OrigR0:         s.OrigR0,
 	}
 }
 

pkg/sentry/arch/syscalls_arm64.go

@@ -50,13 +50,21 @@ func (c *context64) SyscallArgs() SyscallArguments {
 }
 
 // RestartSyscall implements Context.RestartSyscall.
+// Prepare for system call restart, OrigR0 will be restored to R0.
+// Please see the linux code as reference:
+// arch/arm64/kernel/signal.c:do_signal()
 func (c *context64) RestartSyscall() {
 	c.Regs.Pc -= SyscallWidth
-	c.Regs.Regs[8] = uint64(restartSyscallNr)
+	// R0 will be backed up into OrigR0 when entering doSyscall().
+	// Please see the linux code as reference:
+	// arch/arm64/kernel/syscall.c:el0_svc_common().
+	// Here we restore it back.
+	c.Regs.Regs[0] = uint64(c.OrigR0)
 }
 
 // RestartSyscallWithRestartBlock implements Context.RestartSyscallWithRestartBlock.
 func (c *context64) RestartSyscallWithRestartBlock() {
 	c.Regs.Pc -= SyscallWidth
+	c.Regs.Regs[0] = uint64(c.OrigR0)
 	c.Regs.Regs[8] = uint64(restartSyscallNr)
 }