Re: [PATCH v4 1/2] x86, lib: Add WBNOINVD helper functions

Kirill A. Shutemov
2025-01-22T07:32:49Z

On Wed, Jan 22, 2025 at 01:34:37AM +0000, Kevin Loughlin wrote:
> In line with WBINVD usage, add WBONINVD helper functions. For the
> wbnoinvd() helper, fall back to WBINVD if X86_FEATURE_WBNOINVD is not
> present.
>
> Signed-off-by: Kevin Loughlin <kevinloughlin@google.com>
> ---
> arch/x86/include/asm/smp.h | 7 +++++++
> arch/x86/include/asm/special_insns.h | 15 ++++++++++++++-
> arch/x86/lib/cache-smp.c | 12 ++++++++++++
> 3 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> index ca073f40698f..ecf93a243b83 100644
> --- a/arch/x86/include/asm/smp.h
> +++ b/arch/x86/include/asm/smp.h
> @@ -112,6 +112,7 @@ void native_play_dead(void);
> void play_dead_common(void);
> void wbinvd_on_cpu(int cpu);
> int wbinvd_on_all_cpus(void);
> +int wbnoinvd_on_all_cpus(void);
>
> void smp_kick_mwait_play_dead(void);
>
> @@ -160,6 +161,12 @@ static inline int wbinvd_on_all_cpus(void)
> return 0;
> }
>
> +static inline int wbnoinvd_on_all_cpus(void)
> +{
> + wbnoinvd();
> + return 0;
> +}
> +
> static inline struct cpumask *cpu_llc_shared_mask(int cpu)
> {
> return (struct cpumask *)cpumask_of(0);
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 03e7c2d49559..94640c3491d7 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -117,7 +117,20 @@ static inline void wrpkru(u32 pkru)
>
> static __always_inline void wbinvd(void)
> {
> - asm volatile("wbinvd": : :"memory");
> + asm volatile("wbinvd" : : : "memory");
> +}
> +
> +/*
> + * Cheaper version of wbinvd(). Call when caches
> + * need to be written back but not invalidated.
> + */
> +static __always_inline void wbnoinvd(void)
> +{
> + /*
> + * Use the compatible but more destructive "invalidate"
> + * variant when no-invalidate is unavailable.
> + */
> + alternative("wbinvd", "wbnoinvd", X86_FEATURE_WBNOINVD);

The minimal version of binutils kernel supports is 2.25 which doesn't
know about WBNOINVD.

I think you need to do something like.

alternative("wbinvd", ".byte 0xf3; wbinvd", X86_FEATURE_WBNOINVD);

Or propose to bump minimal binutils version.

--
Kiryl Shutsemau / Kirill A. Shutemov