Re: [PATCH v3 05/11] riscv: Implement arch_cmpxchg128() using Zacas

[Andrew Jones]

On Wed, Jul 17, 2024 at 08:19:51AM GMT, Alexandre Ghiti wrote:
> Now that Zacas is supported in the kernel, let's use the double word
> atomic version of amocas to improve the SLUB allocator.
> 
> Note that we have to select fixed registers, otherwise gcc fails to pick
> even registers and then produces a reserved encoding which fails to
> assemble.

Oh, that's quite unfortunate... I guess we should try to get some new
RISC-V inline assembly register constraints added to support register
pairs.

> 
> Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> ---
>  arch/riscv/Kconfig               |  1 +
>  arch/riscv/include/asm/cmpxchg.h | 39 ++++++++++++++++++++++++++++++++
>  2 files changed, 40 insertions(+)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d3b0f92f92da..0bbaec0444d0 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -104,6 +104,7 @@ config RISCV
>  	select GENERIC_VDSO_TIME_NS if HAVE_GENERIC_VDSO
>  	select HARDIRQS_SW_RESEND
>  	select HAS_IOPORT if MMU
> +	select HAVE_ALIGNED_STRUCT_PAGE
>  	select HAVE_ARCH_AUDITSYSCALL
>  	select HAVE_ARCH_HUGE_VMALLOC if HAVE_ARCH_HUGE_VMAP
>  	select HAVE_ARCH_HUGE_VMAP if MMU && 64BIT
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index 97b24da38897..608d98522557 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -289,4 +289,43 @@ end:;									\
>  	arch_cmpxchg_release((ptr), (o), (n));				\
>  })
>  
> +#ifdef CONFIG_RISCV_ISA_ZACAS

This is also 64-bit only, so needs a CONFIG_64BIT check too.

> +
> +#define system_has_cmpxchg128()						\
> +			riscv_has_extension_unlikely(RISCV_ISA_EXT_ZACAS)

nit: let's let this stick out since we have 100 chars

> +
> +union __u128_halves {
> +	u128 full;
> +	struct {
> +		u64 low, high;

Should we consider big endian too?

> +	};
> +};
> +
> +#define __arch_cmpxchg128(p, o, n, cas_sfx)					\
> +({										\
> +	__typeof__(*(p)) __o = (o);						\
> +	union __u128_halves __hn = { .full = (n) };				\
> +	union __u128_halves __ho = { .full = (__o) };				\
> +	register unsigned long x6 asm ("x6") = __hn.low;			\
> +	register unsigned long x7 asm ("x7") = __hn.high;			\
> +	register unsigned long x28 asm ("x28") = __ho.low;			\
> +	register unsigned long x29 asm ("x29") = __ho.high;			\

Can we use t1,t2,t3,t4 rather than the x names?

> +										\
> +	__asm__ __volatile__ (							\
> +		"	amocas.q" cas_sfx " %0, %z3, %2"			\
> +		: "+&r" (x28), "+&r" (x29), "+A" (*(p))				\
> +		: "rJ" (x6), "rJ" (x7)						\
> +		: "memory");							\
> +										\
> +	((u128)x29 << 64) | x28;						\
> +})
> +
> +#define arch_cmpxchg128(ptr, o, n)						\
> +	__arch_cmpxchg128((ptr), (o), (n), ".aqrl")
> +
> +#define arch_cmpxchg128_local(ptr, o, n)					\
> +	__arch_cmpxchg128((ptr), (o), (n), "")
> +
> +#endif /* CONFIG_RISCV_ISA_ZACAS */
> +
>  #endif /* _ASM_RISCV_CMPXCHG_H */
> -- 
> 2.39.2

Thanks,
drew