Re: [PATCH v6 1/2] x86, lib: Add WBNOINVD helper functions

[Tom Lendacky <thomas.lendacky@amd.com>]

On 1/31/25 18:02, Kevin Loughlin wrote:
> In line with WBINVD usage, add WBONINVD helper functions. For the
> wbnoinvd() helper, fall back to WBINVD if via alternative() if
> X86_FEATURE_WBNOINVD is not present. alternative() ensures
> compatibility with early boot code if needed.
> 
> Signed-off-by: Kevin Loughlin <kevinloughlin@google.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>  arch/x86/include/asm/smp.h           |  7 +++++++
>  arch/x86/include/asm/special_insns.h | 19 ++++++++++++++++++-
>  arch/x86/lib/cache-smp.c             | 12 ++++++++++++
>  3 files changed, 37 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..86a903742139 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -117,7 +117,24 @@ static inline void wrpkru(u32 pkru)
>  
>  static __always_inline void wbinvd(void)
>  {
> -	asm volatile("wbinvd": : :"memory");
> +	asm volatile("wbinvd" : : : "memory");
> +}
> +
> +/* Instruction encoding provided for binutils backwards compatibility. */
> +#define WBNOINVD ".byte 0xf3,0x0f,0x09"
> +
> +/*
> + * Cheaper version of wbinvd(). Call when caches
> + * need to be written back but not invalidated.
> + */
> +static __always_inline void wbnoinvd(void)
> +{
> +	/*
> +	 * If WBNOINVD is unavailable, fall back to the compatible but
> +	 * more destructive WBINVD (which still writes the caches back
> +	 * but also invalidates them).
> +	 */
> +	alternative("wbinvd", WBNOINVD, X86_FEATURE_WBNOINVD);
>  }
>  
>  static inline unsigned long __read_cr4(void)
> diff --git a/arch/x86/lib/cache-smp.c b/arch/x86/lib/cache-smp.c
> index 7af743bd3b13..7ac5cca53031 100644
> --- a/arch/x86/lib/cache-smp.c
> +++ b/arch/x86/lib/cache-smp.c
> @@ -20,3 +20,15 @@ int wbinvd_on_all_cpus(void)
>  	return 0;
>  }
>  EXPORT_SYMBOL(wbinvd_on_all_cpus);
> +
> +static void __wbnoinvd(void *dummy)
> +{
> +	wbnoinvd();
> +}
> +
> +int wbnoinvd_on_all_cpus(void)
> +{
> +	on_each_cpu(__wbnoinvd, NULL, 1);
> +	return 0;
> +}
> +EXPORT_SYMBOL(wbnoinvd_on_all_cpus);