Re: [RFC PATCH v2 11/13] powerpc/syscall: Avoid stack frame in likely part of syscall_call_exception()

[Nicholas Piggin <npiggin@gmail.com>]

Christophe Leroy's on April 6, 2020 3:44 am:
> When r3 is not modified, reload it from regs->orig_r3 to free
> volatile registers. This avoids a stack frame for the likely part
> of syscall_call_exception()
> 
> Before : 353 cycles on null_syscall
> After  : 347 cycles on null_syscall
> 
> Before the patch:
> 
> c000b4d4 <system_call_exception>:
> c000b4d4:	7c 08 02 a6 	mflr    r0
> c000b4d8:	94 21 ff e0 	stwu    r1,-32(r1)
> c000b4dc:	93 e1 00 1c 	stw     r31,28(r1)
> c000b4e0:	90 01 00 24 	stw     r0,36(r1)
> c000b4e4:	90 6a 00 88 	stw     r3,136(r10)
> c000b4e8:	81 6a 00 84 	lwz     r11,132(r10)
> c000b4ec:	69 6b 00 02 	xori    r11,r11,2
> c000b4f0:	55 6b ff fe 	rlwinm  r11,r11,31,31,31
> c000b4f4:	0f 0b 00 00 	twnei   r11,0
> c000b4f8:	81 6a 00 a0 	lwz     r11,160(r10)
> c000b4fc:	55 6b 07 fe 	clrlwi  r11,r11,31
> c000b500:	0f 0b 00 00 	twnei   r11,0
> c000b504:	7c 0c 42 e6 	mftb    r0
> c000b508:	83 e2 00 08 	lwz     r31,8(r2)
> c000b50c:	81 82 00 28 	lwz     r12,40(r2)
> c000b510:	90 02 00 24 	stw     r0,36(r2)
> c000b514:	7d 8c f8 50 	subf    r12,r12,r31
> c000b518:	7c 0c 02 14 	add     r0,r12,r0
> c000b51c:	90 02 00 08 	stw     r0,8(r2)
> c000b520:	7c 10 13 a6 	mtspr   80,r0
> c000b524:	81 62 00 70 	lwz     r11,112(r2)
> c000b528:	71 60 86 91 	andi.   r0,r11,34449
> c000b52c:	40 82 00 34 	bne     c000b560 <system_call_exception+0x8c>
> c000b530:	2b 89 01 b6 	cmplwi  cr7,r9,438
> c000b534:	41 9d 00 64 	bgt     cr7,c000b598 <system_call_exception+0xc4>
> c000b538:	3d 40 c0 5c 	lis     r10,-16292
> c000b53c:	55 29 10 3a 	rlwinm  r9,r9,2,0,29
> c000b540:	39 4a 41 e8 	addi    r10,r10,16872
> c000b544:	80 01 00 24 	lwz     r0,36(r1)
> c000b548:	7d 2a 48 2e 	lwzx    r9,r10,r9
> c000b54c:	7c 08 03 a6 	mtlr    r0
> c000b550:	7d 29 03 a6 	mtctr   r9
> c000b554:	83 e1 00 1c 	lwz     r31,28(r1)
> c000b558:	38 21 00 20 	addi    r1,r1,32
> c000b55c:	4e 80 04 20 	bctr
> 
> After the patch:
> 
> c000b4d4 <system_call_exception>:
> c000b4d4:	81 6a 00 84 	lwz     r11,132(r10)
> c000b4d8:	90 6a 00 88 	stw     r3,136(r10)
> c000b4dc:	69 6b 00 02 	xori    r11,r11,2
> c000b4e0:	55 6b ff fe 	rlwinm  r11,r11,31,31,31
> c000b4e4:	0f 0b 00 00 	twnei   r11,0
> c000b4e8:	80 6a 00 a0 	lwz     r3,160(r10)
> c000b4ec:	54 63 07 fe 	clrlwi  r3,r3,31
> c000b4f0:	0f 03 00 00 	twnei   r3,0
> c000b4f4:	7d 6c 42 e6 	mftb    r11
> c000b4f8:	81 82 00 08 	lwz     r12,8(r2)
> c000b4fc:	80 02 00 28 	lwz     r0,40(r2)
> c000b500:	91 62 00 24 	stw     r11,36(r2)
> c000b504:	7c 00 60 50 	subf    r0,r0,r12
> c000b508:	7d 60 5a 14 	add     r11,r0,r11
> c000b50c:	91 62 00 08 	stw     r11,8(r2)
> c000b510:	7c 10 13 a6 	mtspr   80,r0
> c000b514:	80 62 00 70 	lwz     r3,112(r2)
> c000b518:	70 6b 86 91 	andi.   r11,r3,34449
> c000b51c:	40 82 00 28 	bne     c000b544 <system_call_exception+0x70>
> c000b520:	2b 89 01 b6 	cmplwi  cr7,r9,438
> c000b524:	41 9d 00 84 	bgt     cr7,c000b5a8 <system_call_exception+0xd4>
> c000b528:	80 6a 00 88 	lwz     r3,136(r10)
> c000b52c:	3d 40 c0 5c 	lis     r10,-16292
> c000b530:	55 29 10 3a 	rlwinm  r9,r9,2,0,29
> c000b534:	39 4a 41 e4 	addi    r10,r10,16868
> c000b538:	7d 2a 48 2e 	lwzx    r9,r10,r9
> c000b53c:	7d 29 03 a6 	mtctr   r9
> c000b540:	4e 80 04 20 	bctr
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> ---
>  arch/powerpc/kernel/syscall.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index 69d75fc4a5eb..630c423e089a 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -91,6 +91,8 @@ notrace long system_call_exception(long r3, long r4, long r5,
>  
>  	} else if (unlikely(r0 >= NR_syscalls)) {
>  		return -ENOSYS;
> +	} else {
> +		r3 = regs->orig_gpr3;
>  	}

So this just gives enough volatiles to avoid spilling to stack? I wonder
about other various options here if they would cause a spill anyway.

Interesting optimisation, it would definitely need a comment. Would be
nice if we had a way to tell the compiler that a local can be reloaded
from a particular address.

Thanks,
Nick