Loongarch: optimize syscall reg save

deepin inclusion
category: performance

It saves a st.d in the hot syscall path, and let
the compiler know to optimize it in asm,
and helps to improve the syscall performance little.

Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: opsiff

arch/loongarch/kernel/entry.S

@@ -30,7 +30,8 @@ SYM_CODE_START(handle_syscall)
 	addi.d		sp, sp, -PT_SIZE
 	cfi_st		t2, PT_R3
 	cfi_rel_offset	sp, PT_R3
-	st.d		zero, sp, PT_R0
+	# Note it will be set in do_syscall regs->regs[0] = 0;
+	# st.d		zero, sp, PT_R0
 	csrrd		t2, LOONGARCH_CSR_PRMD
 	st.d		t2, sp, PT_PRMD
 	csrrd		t2, LOONGARCH_CSR_CRMD

arch/loongarch/kernel/syscall.c

@@ -44,6 +44,8 @@ void noinstr do_syscall(struct pt_regs *regs)
 	sys_call_fn syscall_fn;
 
 	nr = regs->regs[11];
+	// Move from handle_syscall macro to save a memio
+	regs->regs[0] = 0;
 	/* Set for syscall restarting */
 	if (nr < NR_syscalls)
 		regs->regs[0] = nr + 1;