powerpc: Change system_call_exception calling convention

Change system_call_exception arguments to pass a pointer to a stack
frame container caller state, as well as the original r0, which
determines the number of the syscall. This has been observed to yield
improved performance to passing them by registers, circumventing the
need to allocate a stack frame.

Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Retain clearing of high bits of args for compat tasks]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220921065605.1051927-21-rmclure@linux.ibm.com

Author: rohanmclure

arch/powerpc/include/asm/interrupt.h

@@ -665,8 +665,7 @@ static inline void interrupt_cond_local_irq_enable(struct pt_regs *regs)
 		local_irq_enable();
 }
 
-long system_call_exception(long r3, long r4, long r5, long r6, long r7, long r8,
-			   unsigned long r0, struct pt_regs *regs);
+long system_call_exception(struct pt_regs *regs, unsigned long r0);
 notrace unsigned long syscall_exit_prepare(unsigned long r3, struct pt_regs *regs, long scv);
 notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs);
 notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs);

arch/powerpc/kernel/entry_32.S

@@ -122,9 +122,9 @@ transfer_to_syscall:
 	SAVE_NVGPRS(r1)
 	kuep_lock
 
-	/* Calling convention has r9 = orig r0, r10 = regs */
-	addi	r10,r1,STACK_FRAME_OVERHEAD
-	mr	r9,r0
+	/* Calling convention has r3 = regs, r4 = orig r0 */
+	addi	r3,r1,STACK_FRAME_OVERHEAD
+	mr	r4,r0
 	bl	system_call_exception
 
 ret_from_syscall:

arch/powerpc/kernel/interrupt_64.S

@@ -60,7 +60,7 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_\name)
 	ld	r2,PACATOC(r13)
 	mfcr	r12
 	li	r11,0
-	/* Can we avoid saving r3-r8 in common case? */
+	/* Save syscall parameters in r3-r8 */
 	SAVE_GPRS(3, 8, r1)
 	/* Zero r9-r12, this should only be required when restoring all GPRs */
 	std	r11,GPR9(r1)
@@ -77,9 +77,11 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_\name)
 	std	r11,_TRAP(r1)
 	std	r12,_CCR(r1)
 	std	r3,ORIG_GPR3(r1)
-	addi	r10,r1,STACK_FRAME_OVERHEAD
+	/* Calling convention has r3 = regs, r4 = orig r0 */
+	addi	r3,r1,STACK_FRAME_OVERHEAD
+	mr	r4,r0
 	ld	r11,exception_marker@toc(r2)
-	std	r11,-16(r10)		/* "regshere" marker */
+	std	r11,-16(r3)		/* "regshere" marker */
 
 BEGIN_FTR_SECTION
 	HMT_MEDIUM
@@ -94,8 +96,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 	 * but this is the best we can do.
 	 */
 
-	/* Calling convention has r9 = orig r0, r10 = regs */
-	mr	r9,r0
 	bl	system_call_exception
 
 .Lsyscall_vectored_\name\()_exit:
@@ -227,7 +227,7 @@ END_BTB_FLUSH_SECTION
 	ld	r2,PACATOC(r13)
 	mfcr	r12
 	li	r11,0
-	/* Can we avoid saving r3-r8 in common case? */
+	/* Save syscall parameters in r3-r8 */
 	SAVE_GPRS(3, 8, r1)
 	/* Zero r9-r12, this should only be required when restoring all GPRs */
 	std	r11,GPR9(r1)
@@ -250,9 +250,11 @@ END_BTB_FLUSH_SECTION
 	std	r11,_TRAP(r1)
 	std	r12,_CCR(r1)
 	std	r3,ORIG_GPR3(r1)
-	addi	r10,r1,STACK_FRAME_OVERHEAD
+	/* Calling convention has r3 = regs, r4 = orig r0 */
+	addi	r3,r1,STACK_FRAME_OVERHEAD
+	mr	r4,r0
 	ld	r11,exception_marker@toc(r2)
-	std	r11,-16(r10)		/* "regshere" marker */
+	std	r11,-16(r3)		/* "regshere" marker */
 
 #ifdef CONFIG_PPC_BOOK3S
 	li	r11,1
@@ -273,8 +275,6 @@ END_BTB_FLUSH_SECTION
 	wrteei	1
 #endif
 
-	/* Calling convention has r9 = orig r0, r10 = regs */
-	mr	r9,r0
 	bl	system_call_exception
 
 .Lsyscall_exit:

arch/powerpc/kernel/syscall.c

@@ -13,10 +13,9 @@
 
 
 /* Has to run notrace because it is entered not completely "reconciled" */
-notrace long system_call_exception(long r3, long r4, long r5,
-				   long r6, long r7, long r8,
-				   unsigned long r0, struct pt_regs *regs)
+notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
 {
+	unsigned long r3, r4, r5, r6, r7, r8;
 	long ret;
 	syscall_fn f;
 
@@ -136,12 +135,6 @@ notrace long system_call_exception(long r3, long r4, long r5,
 		r0 = do_syscall_trace_enter(regs);
 		if (unlikely(r0 >= NR_syscalls))
 			return regs->gpr[3];
-		r3 = regs->gpr[3];
-		r4 = regs->gpr[4];
-		r5 = regs->gpr[5];
-		r6 = regs->gpr[6];
-		r7 = regs->gpr[7];
-		r8 = regs->gpr[8];
 
 	} else if (unlikely(r0 >= NR_syscalls)) {
 		if (unlikely(trap_is_unsupported_scv(regs))) {
@@ -152,6 +145,13 @@ notrace long system_call_exception(long r3, long r4, long r5,
 		return -ENOSYS;
 	}
 
+	r3 = regs->gpr[3];
+	r4 = regs->gpr[4];
+	r5 = regs->gpr[5];
+	r6 = regs->gpr[6];
+	r7 = regs->gpr[7];
+	r8 = regs->gpr[8];
+
 	/* May be faster to do array_index_nospec? */
 	barrier_nospec();