Re: [PATCH 12/12] [PROBABLY WRONG] s390: void '0' constraint in inline assembly

Nick Desaulniers
2019-04-10T16:35:02Z

On Wed, Apr 10, 2019 at 6:55 AM Martin Schwidefsky
<schwidefsky@de.ibm.com> wrote:
>
> On Mon, 8 Apr 2019 23:26:25 +0200
> Arnd Bergmann <arnd@arndb.de> wrote:
>
> > clang does not understand the contraint "0" in the CALL_ON_STACK()
> > macro:
> >
> > ../arch/s390/mm/maccess.c:117:10: error: invalid input constraint '0' in asm
> > return CALL_ON_STACK(_memcpy_real, S390_lowcore.nodat_stack,
> > ^
> > ../arch/s390/include/asm/processor.h:292:20: note: expanded from macro 'CALL_ON_STACK'
> > [_fn] "X" (fn) CALL_FMT_##nr : CALL_CLOBBER_##nr); \
> > ^
> > <scratch space>:207:1: note: expanded from here
> > CALL_FMT_3
> > ^
> > ../arch/s390/include/asm/processor.h:267:20: note: expanded from macro 'CALL_FMT_3'
> > #define CALL_FMT_3 CALL_FMT_2, "d" (r4)
> > ^
> > ../arch/s390/include/asm/processor.h:266:20: note: expanded from macro 'CALL_FMT_2'
> > #define CALL_FMT_2 CALL_FMT_1, "d" (r3)
> > ^
> > ../arch/s390/include/asm/processor.h:265:32: note: expanded from macro 'CALL_FMT_1'
> > #define CALL_FMT_1 CALL_FMT_0, "0" (r2)
> > ^
> >
> > I don't know what the correct fix here would be, changing it to "d" made
> > it build, since clang does understand this one.
> >
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> > arch/s390/include/asm/processor.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/s390/include/asm/processor.h b/arch/s390/include/asm/processor.h
> > index 700c650ffd4f..84c59c99668a 100644
> > --- a/arch/s390/include/asm/processor.h
> > +++ b/arch/s390/include/asm/processor.h
> > @@ -262,7 +262,7 @@ static __no_kasan_or_inline unsigned short stap(void)
> > register unsigned long r4 asm("6") = (unsigned long)(arg5)
> >
> > #define CALL_FMT_0
> > -#define CALL_FMT_1 CALL_FMT_0, "0" (r2)
> > +#define CALL_FMT_1 CALL_FMT_0, "d" (r2)
> > #define CALL_FMT_2 CALL_FMT_1, "d" (r3)
> > #define CALL_FMT_3 CALL_FMT_2, "d" (r4)
> > #define CALL_FMT_4 CALL_FMT_3, "d" (r5)
>
> This is (slightly) wrong. %r2 is used as the input register for the first argument
> and the result value for the call. With your patch you force the compiler to load
> the first argument in two registers. One solution would be to CALL_FMT1 as
>
> #define CALL_FMT1 CALL_FMT_0
>
> It still is not optimal though as for CALL_FMT_0 the "+&d" (r2) indicates an
> input but CALL_ARGS_0 does not initialize r2.
>
> I am thinking about the following patch to cover all cases:
> --
> From 91a4abbec91a9f26f84f7386f2c0f96de669b0eb Mon Sep 17 00:00:00 2001
> From: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Date: Wed, 10 Apr 2019 15:48:43 +0200
> Subject: [PATCH] s390: fine-tune stack switch helper

Thanks for the patch. Just some minor typos in the commit.

>
> The CALL_ON_STACK helper currently does not work with clang and for
> calls without arguments it does not initialize r2 although the contraint

- calls without arguments it ...
+ calls without arguments. It ...

- although the contraint
+ although the constraint

`:set spell` in vim (I wonder if there's a way to set that when
writing commit messages automatically)

> is "+&d". Rework the CALL_FMT_x and the CALL_ON_STACK macros to work
> with clang and produce optimal code in all cases.
>
> Reported-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> ---
> arch/s390/include/asm/processor.h | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/arch/s390/include/asm/processor.h b/arch/s390/include/asm/processor.h
> index 81038ab357ce..0ee022247580 100644
> --- a/arch/s390/include/asm/processor.h
> +++ b/arch/s390/include/asm/processor.h
> @@ -261,12 +261,12 @@ static __no_kasan_or_inline unsigned short stap(void)
> CALL_ARGS_4(arg1, arg2, arg3, arg4); \
> register unsigned long r4 asm("6") = (unsigned long)(arg5)
>
> -#define CALL_FMT_0
> -#define CALL_FMT_1 CALL_FMT_0, "0" (r2)
> -#define CALL_FMT_2 CALL_FMT_1, "d" (r3)
> -#define CALL_FMT_3 CALL_FMT_2, "d" (r4)
> -#define CALL_FMT_4 CALL_FMT_3, "d" (r5)
> -#define CALL_FMT_5 CALL_FMT_4, "d" (r6)
> +#define CALL_FMT_0 "=&d" (r2) :
> +#define CALL_FMT_1 "+&d" (r2) :
> +#define CALL_FMT_2 CALL_FMT_1 "d" (r3),
> +#define CALL_FMT_3 CALL_FMT_2 "d" (r4),
> +#define CALL_FMT_4 CALL_FMT_3 "d" (r5),
> +#define CALL_FMT_5 CALL_FMT_4 "d" (r6),
>
> #define CALL_CLOBBER_5 "0", "1", "14", "cc", "memory"
> #define CALL_CLOBBER_4 CALL_CLOBBER_5
> @@ -286,10 +286,10 @@ static __no_kasan_or_inline unsigned short stap(void)
> " stg %[_prev],%[_bc](15)\n" \
> " brasl 14,%[_fn]\n" \
> " la 15,0(%[_prev])\n" \
> - : "+&d" (r2), [_prev] "=&a" (prev) \
> - : [_stack] "a" (stack), \
> + : [_prev] "=&a" (prev), CALL_FMT_##nr \
> + [_stack] "a" (stack), \
> [_bc] "i" (offsetof(struct stack_frame, back_chain)), \
> - [_fn] "X" (fn) CALL_FMT_##nr : CALL_CLOBBER_##nr); \
> + [_fn] "X" (fn) : CALL_CLOBBER_##nr); \
> r2; \
> })
>
> --
> 2.16.4
> --
> blue skies,
> Martin.
>
> "Reality continues to ruin my life." - Calvin.
>
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--
Thanks,
~Nick Desaulniers