linux-kernel - Re: [PATCH v4 1/6] x86: KVM: Advertise CMPccXADD CPUID to user space

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Message-ID: <ffc31149-8780-5499-7d25-08b871399bde@linux.intel.com>
Date:   Wed, 23 Nov 2022 15:46:57 +0800
From:   Jiaxi Chen <jiaxi.chen@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...el.com>
Cc:     kvm@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        seanjc@...gle.com, pbonzini@...hat.com, ndesaulniers@...gle.com,
        alexandre.belloni@...tlin.com, peterz@...radead.org,
        jpoimboe@...nel.org, chang.seok.bae@...el.com,
        pawan.kumar.gupta@...ux.intel.com, babu.moger@....com,
        jmattson@...gle.com, sandipan.das@....com, tony.luck@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, fenghua.yu@...el.com,
        keescook@...omium.org, nathan@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/6] x86: KVM: Advertise CMPccXADD CPUID to user space

On 11/21/2022 11:38 PM, Borislav Petkov wrote:
> On Mon, Nov 21, 2022 at 10:46:21PM +0800, Jiaxi Chen wrote:
>> Features which has been enabled in kernel usually should be added to
>> /proc/cpuinfo.
> 
> No, pls read this first: Documentation/x86/cpuinfo.rst
> 
> If something's not clear, we will extend it so that it is.
> 
> /proc/cpuinfo - a user ABI - is not a dumping ground for CPUID bits.
> 

Thanks. Sorry for the miss understanding.

For those feature bits who have truly kernel usage, their flags should
appear in /proc/cpuinfo. For others, they are not generally show up
here, it depends.

As for features in this patch series:

The first-way defined bits are on an expected-dense cpuid leaf[1] and
some of their siblings have kernel usages[2]. Given that, define them
like X86_FEATURE_* in arch/x86/include/asm/cpufeatures.h. But due to
their complicated and unreadable feature name[3], prefer to hide them in
/proc/cpuinfo.

The second-way defined bits are on a new and sparse cpuid leaf. Besides,
these bits have no turly kernel use case. Therefore, move these new bits
to kvm-only leaves to achieve the purpose for advertising these bits to
kvm userspace[4]. Then of course they will not show up in /proc/cpuinfo.

[1] https://lore.kernel.org/all/Y3O7UYWfOLfJkwM%2F@zn.tnic/
[2]
https://lore.kernel.org/all/f8607d23-afaa-2670-dd03-2ae8ec1e79a0@intel.com/
[3]
https://lore.kernel.org/all/6d7fae50-ef3c-dc1e-336c-691095007117@intel.com/
[4] https://lore.kernel.org/all/Y1ATKF2xjERFbspn@google.com/

-- 
Regards,
Jiaxi