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

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Message-ID: <d2dce9d8-b79e-7d83-15a5-68889b140229@amd.com>
Date: Wed, 22 Jan 2025 13:39:19 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 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, 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 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@...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);

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

Thanks,
Tom

> 
> Or propose to bump minimal binutils version.
>