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

Tom Lendacky
2025-01-22T19:39:24Z

On 1/22/25 01:32, Kirill A. Shutemov wrote:
> 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);

I think "rep; wbinvd" would work as well.

Thanks,
Tom

>
> Or propose to bump minimal binutils version.
>