xtensa: expose syscall through user_pt_regs

Use one of the reserved slots in struct user_pt_regs to return syscall
number in the GPR regset. Update syscall number from the GPR regset only
when it's non-zero.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/uapi/asm/ptrace.h

@@ -50,7 +50,8 @@ struct user_pt_regs {
 	__u32 windowstart;
 	__u32 windowbase;
 	__u32 threadptr;
-	__u32 reserved[7 + 48];
+	__u32 syscall;
+	__u32 reserved[6 + 48];
 	__u32 a[64];
 };
 

arch/xtensa/kernel/ptrace.c

@@ -52,6 +52,7 @@ static int gpr_get(struct task_struct *target,
 		.threadptr = regs->threadptr,
 		.windowbase = regs->windowbase,
 		.windowstart = regs->windowstart,
+		.syscall = regs->syscall,
 	};
 
 	memcpy(newregs.a,
@@ -91,6 +92,9 @@ static int gpr_set(struct task_struct *target,
 	regs->sar = newregs.sar;
 	regs->threadptr = newregs.threadptr;
 
+	if (newregs.syscall)
+		regs->syscall = newregs.syscall;
+
 	if (newregs.windowbase != regs->windowbase ||
 	    newregs.windowstart != regs->windowstart) {
 		u32 rotws, wmask;