Re: [PATCH v3 03/11] riscv: Implement cmpxchg8/16() using Zabha

Conor Dooley
2024-07-17T15:30:37Z

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On Wed, Jul 17, 2024 at 10:26:34AM -0500, Andrew Jones wrote:
> On Wed, Jul 17, 2024 at 08:19:49AM GMT, Alexandre Ghiti wrote:
> > -#define __arch_cmpxchg_masked(sc_sfx, prepend, append, r, p, o, n) \
> > +#define __arch_cmpxchg_masked(sc_sfx, cas_sfx, prepend, append, r, p, o, n) \
> > ({ \
> > + __label__ no_zabha_zacas, end; \
> > + \
> > + if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA) && \
> > + IS_ENABLED(CONFIG_RISCV_ISA_ZACAS)) { \
> > + asm goto(ALTERNATIVE("j %[no_zabha_zacas]", "nop", 0, \
> > + RISCV_ISA_EXT_ZABHA, 1) \
> > + : : : : no_zabha_zacas); \
> > + asm goto(ALTERNATIVE("j %[no_zabha_zacas]", "nop", 0, \
> > + RISCV_ISA_EXT_ZACAS, 1) \
> > + : : : : no_zabha_zacas); \
>
> I came late to the call, but I guess trying to get rid of these asm gotos
> was the topic of the discussion. The proposal was to try and use static
> branches, but keep in mind that we've had trouble with static branches
> inside macros in the past when those macros are used in many places[1]
>
> [1] commit 0b1d60d6dd9e ("riscv: Fix build with CONFIG_CC_OPTIMIZE_FOR_SIZE=y")

The other half of the suggestion was not using an asm goto, but instead
trying to patch the whole thing in the alternative, for the problematic
section with llvm < 17.

>
> > + \
> > + __asm__ __volatile__ ( \
> > + prepend \
> > + " amocas" cas_sfx " %0, %z2, %1\n" \
> > + append \
> > + : "+&r" (r), "+A" (*(p)) \
> > + : "rJ" (n) \
> > + : "memory"); \
> > + goto end; \
> > + } \
> > + \
> > +no_zabha_zacas:; \
>
> unnecessary ;
>
> > u32 *__ptr32b = (u32 *)((ulong)(p) & ~0x3); \
> > ulong __s = ((ulong)(p) & (0x4 - sizeof(*p))) * BITS_PER_BYTE; \
> > ulong __mask = GENMASK(((sizeof(*p)) * BITS_PER_BYTE) - 1, 0) \
> > @@ -133,6 +155,8 @@
> > : "memory"); \
> > \
> > r = (__typeof__(*(p)))((__retx & __mask) >> __s); \
> > + \
> > +end:; \
> > })
> >
> > #define __arch_cmpxchg(lr_sfx, sc_cas_sfx, prepend, append, r, p, co, o, n) \

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