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

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Message-ID: <aomvugehkmfj6oi7bwmtiqfbdyet7zyd2llri3c5rgcmgqjkfz@tslxstgihjb5>
Date: Wed, 22 Jan 2025 09:32:43 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Kevin Loughlin <kevinloughlin@...gle.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	seanjc@...gle.com, pbonzini@...hat.com, kai.huang@...el.com, ubizjak@...il.com, 
	jgross@...e.com, kvm@...r.kernel.org, thomas.lendacky@....com, pgonda@...gle.com, 
	sidtelang@...gle.com, mizhang@...gle.com, rientjes@...gle.com, manalinandan@...gle.com, 
	szy0127@...u.edu.cn
Subject: Re: [PATCH v4 1/2] x86, lib: Add WBNOINVD helper functions

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@...gle.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