xtensa: clean up system_call/xtensa_rt_sigreturn interaction

system_call assembly code always pushes pointer to struct pt_regs as the
last additional parameter for all system calls. The only user of this
feature is xtensa_rt_sigreturn.
Avoid this special case. Define xtensa_rt_sigreturn as accepting no
argiments. Use current_pt_regs to get pointer to struct pt_regs in
xtensa_rt_sigreturn. Don't pass additional parameter from system_call
code.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/syscall.h

@@ -79,7 +79,7 @@ static inline void syscall_set_arguments(struct task_struct *task,
 		regs->areg[reg[i]] = args[i];
 }
 
-asmlinkage long xtensa_rt_sigreturn(struct pt_regs*);
+asmlinkage long xtensa_rt_sigreturn(void);
 asmlinkage long xtensa_shmat(int, char __user *, int);
 asmlinkage long xtensa_fadvise64_64(int, int,
 				    unsigned long long, unsigned long long);

arch/xtensa/kernel/entry.S

@@ -1876,8 +1876,7 @@ ENDPROC(fast_store_prohibited)
 
 ENTRY(system_call)
 
-	/* reserve 4 bytes on stack for function parameter */
-	abi_entry(4)
+	abi_entry_default
 
 	/* regs->syscall = regs->areg[2] */
 
@@ -1915,9 +1914,6 @@ ENTRY(system_call)
 	l32i	a10, a2, PT_AREG8
 	l32i	a11, a2, PT_AREG9
 
-	/* Pass one additional argument to the syscall: pt_regs (on stack) */
-	s32i	a2, a1, 0
-
 	callx4	a4
 
 1:	/* regs->areg[2] = return_value */
@@ -1925,12 +1921,12 @@ ENTRY(system_call)
 	s32i	a6, a2, PT_AREG2
 	bnez	a3, 1f
 .Lsyscall_exit:
-	abi_ret(4)
+	abi_ret_default
 
 1:
 	mov	a6, a2
 	call4	do_syscall_trace_leave
-	abi_ret(4)
+	abi_ret_default
 
 ENDPROC(system_call)
 

arch/xtensa/kernel/signal.c

@@ -236,9 +236,9 @@ restore_sigcontext(struct pt_regs *regs, struct rt_sigframe __user *frame)
  * Do a signal return; undo the signal stack.
  */
 
-asmlinkage long xtensa_rt_sigreturn(long a0, long a1, long a2, long a3,
-				    long a4, long a5, struct pt_regs *regs)
+asmlinkage long xtensa_rt_sigreturn(void)
 {
+	struct pt_regs *regs = current_pt_regs();
 	struct rt_sigframe __user *frame;
 	sigset_t set;
 	int ret;