xtensa: rearrange syscall tracing

system_call saves and restores syscall number across system call to make
clone and execv entry and exit tracing match. This complicates things
when syscall code may be changed by ptrace.
Preserve syscall code in copy_thread and start_thread directly instead of
doing tricks in system_call.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/processor.h

@@ -195,6 +195,7 @@ struct thread_struct {
 /* Clearing a0 terminates the backtrace. */
 #define start_thread(regs, new_pc, new_sp) \
 	do { \
+		unsigned long syscall = (regs)->syscall; \
 		memset((regs), 0, sizeof(*(regs))); \
 		(regs)->pc = (new_pc); \
 		(regs)->ps = USER_PS_VALUE; \
@@ -204,7 +205,7 @@ struct thread_struct {
 		(regs)->depc = 0; \
 		(regs)->windowbase = 0; \
 		(regs)->windowstart = 1; \
-		(regs)->syscall = NO_SYSCALL; \
+		(regs)->syscall = syscall; \
 	} while (0)
 
 /* Forward declaration */

arch/xtensa/kernel/entry.S

@@ -1895,8 +1895,6 @@ ENTRY(system_call)
 	l32i	a7, a2, PT_SYSCALL
 
 1:
-	s32i	a7, a1, 4
-
 	/* syscall = sys_call_table[syscall_nr] */
 
 	movi	a4, sys_call_table
@@ -1930,12 +1928,8 @@ ENTRY(system_call)
 	abi_ret(4)
 
 1:
-	l32i	a4, a1, 4
-	l32i	a3, a2, PT_SYSCALL
-	s32i	a4, a2, PT_SYSCALL
 	mov	a6, a2
 	call4	do_syscall_trace_leave
-	s32i	a3, a2, PT_SYSCALL
 	abi_ret(4)
 
 ENDPROC(system_call)

arch/xtensa/kernel/process.c

@@ -264,6 +264,8 @@ int copy_thread(unsigned long clone_flags, unsigned long usp_thread_fn,
 			       &regs->areg[XCHAL_NUM_AREGS - len/4], len);
 		}
 
+		childregs->syscall = regs->syscall;
+
 		/* The thread pointer is passed in the '4th argument' (= a5) */
 		if (clone_flags & CLONE_SETTLS)
 			childregs->threadptr = childregs->areg[5];