Re: [v3, 1/9] selftests/powerpc: Test the preservation of FPU and VMX regs across syscall

[Michael Ellerman <mpe@ellerman.id.au>]

[offlist]

On Thu, 2016-21-01 at 00:55:41 UTC, Cyril Bur wrote:
> Test that the non volatile floating point and Altivec registers get
> correctly preserved across the fork() syscall.
> 
> fork() works nicely for this purpose, the registers should be the same for
> both parent and child

...

> diff --git a/tools/testing/selftests/powerpc/basic_asm.h b/tools/testing/selftests/powerpc/basic_asm.h
> new file mode 100644
> index 0000000..27aca79
> --- /dev/null
> +++ b/tools/testing/selftests/powerpc/basic_asm.h
> @@ -0,0 +1,26 @@
> +#include <ppc-asm.h>
> +#include <asm/unistd.h>
> +
> +#define LOAD_REG_IMMEDIATE(reg,expr) \
> +	lis	reg,(expr)@highest;	\
> +	ori	reg,reg,(expr)@higher;	\
> +	rldicr	reg,reg,32,31;	\
> +	oris	reg,reg,(expr)@high;	\
> +	ori	reg,reg,(expr)@l;
> +
> +#define PUSH_BASIC_STACK(size) \
> +	std	2,24(sp); \
> +	mflr	r0; \
> +	std	r0,16(sp); \
> +	mfcr	r0; \
> +	stw	r0,8(sp); \
> +	stdu	sp,-size(sp);

Unless I'm completely misreading this, that's not ABI compliant.

You are supposed to save LR into your caller's stack frame, but the TOC and CR
go into *your* stack frame.

I know that's not super obvious from the ABI doc.

See for example this code generated by GCC:

extern int global_thing;

int foo(int x, int (*fp)(int x))
{
	int a, b, c;

	a = x;
	b = a + global_thing;
	c = b + 2;

	a = fp(x);

	printf("%d %d %d\n", a, b, c);

	return a;
}

0000000000000000 <foo>:
   0:	00 00 4c 3c 	addis   r2,r12,0
			0: R_PPC64_REL16_HA	.TOC.
   4:	00 00 42 38 	addi    r2,r2,0
			4: R_PPC64_REL16_LO	.TOC.+0x4
   8:	a6 02 08 7c 	mflr    r0
   c:	f0 ff c1 fb 	std     r30,-16(r1)	<- save r30 into (what will be) our stack frame
  10:	f8 ff e1 fb 	std     r31,-8(r1)	<- save r31 into (what will be) our stack frame
  14:	78 23 8c 7c 	mr      r12,r4		<- put fp into r12 because we'll be calling its global entry point
  18:	a6 03 89 7c 	mtctr   r4		<- put fp into CTR
  1c:	00 00 22 3d 	addis   r9,r2,0
			1c: R_PPC64_TOC16_HA	.toc
  20:	00 00 29 e9 	ld      r9,0(r9)
			20: R_PPC64_TOC16_LO_DS	.toc
  24:	10 00 01 f8 	std     r0,16(r1)	<- save r0 (LR) into *our caller's* stack frame
  28:	91 ff 21 f8 	stdu    r1,-112(r1)	<- create our stack frame & store the backchain pointer
  2c:	18 00 41 f8 	std     r2,24(r1)	<- save r2 (TOC) into *our* stack frame
  30:	00 00 e9 83 	lwz     r31,0(r9)
  34:	14 1a ff 7f 	add     r31,r31,r3
  38:	21 04 80 4e 	bctrl			<- call fp, it will get it's TOC pointer using r12
  3c:	18 00 41 e8 	ld      r2,24(r1)	<- restore our r2 (TOC) from *our* stack frame
  40:	02 00 ff 38 	addi    r7,r31,2
  44:	b4 07 e6 7f 	extsw   r6,r31
  48:	b4 07 e7 7c 	extsw   r7,r7
  4c:	78 1b 7e 7c 	mr      r30,r3
  50:	00 00 82 3c 	addis   r4,r2,0
			50: R_PPC64_TOC16_HA	.rodata.str1.8
  54:	78 f3 c5 7f 	mr      r5,r30
  58:	00 00 84 38 	addi    r4,r4,0
			58: R_PPC64_TOC16_LO	.rodata.str1.8
  5c:	01 00 60 38 	li      r3,1
  60:	01 00 00 48 	bl      60 <foo+0x60>
			60: R_PPC64_REL24	__printf_chk
  64:	00 00 00 60 	nop
  68:	70 00 21 38 	addi    r1,r1,112	<- pop our stack frame
  6c:	78 f3 c3 7f 	mr      r3,r30
  70:	10 00 01 e8 	ld      r0,16(r1)	<- restore r0 (LR) from our caller's stack frame
  74:	f0 ff c1 eb 	ld      r30,-16(r1)	<- restore r30 from (what was) our stack frame
  78:	f8 ff e1 eb 	ld      r31,-8(r1)	<- restore r31 from (what was) our stack frame
  7c:	a6 03 08 7c 	mtlr    r0		<- restore LR
  80:	20 00 80 4e 	blr			<- return



So your macro wants to be more like:

#define PUSH_BASIC_STACK(_size) \
	mflr	r0;		\
	std	r0,16(r1);	\
	stdu	r1,-_size(r1);	\
	mfcr	r0;		\
	stw	r0,8(r1);	\
	std	r2,24(r1);

Though untested.

Also you're relying on the caller to ensure size is big enough. So maybe it'd
be better if you ensured that, making the size parameter the *extra* size above
the minimum stack frame. eg:


#define PUSH_BASIC_STACK(_extra) 	\
	mflr	r0;			\
	std	r0,16(r1);		\
	stdu	r1,-(_extra + 32)(r1);	\
	mfcr	r0;			\
	stw	r0,8(r1);		\
	std	r2,24(r1);


cheers