linux-kernel - Re: [PATCH] x86/cpufeatures: Add CPUID_LNX_5 to track recently added Linux-defined word

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Date: Thu, 4 Apr 2024 17:33:06 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 Sandipan Das <sandipan.das@....com>
Subject: Re: [PATCH] x86/cpufeatures: Add CPUID_LNX_5 to track recently added
 Linux-defined word

On 4/4/24 17:16, Sean Christopherson wrote:
> Add CPUID_LNX_5 to track cpufeatures' word 21, and add the appropriate
> compile-time assert in KVM to prevent direct lookups on the features in
> CPUID_LNX_5.  KVM uses X86_FEATURE_* flags to manage guest CPUID, and so
> must translate features that are scattered by Linux from the Linux-defined
> bit to the hardware-defined bit, i.e. should never try to directly access
> scattered features in guest CPUID.
> 
> Opportunistically add NR_CPUID_WORDS to enum cpuid_leafs, along with a
> compile-time assert in KVM's CPUID infrastructure to ensure that future
> additions update cpuid_leafs along with NCAPINTS.
> 
> No functional change intended.
> 
> Fixes: 7f274e609f3d ("x86/cpufeatures: Add new word for scattered features")
> Cc: Sandipan Das <sandipan.das@....com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

I don't think I noticed this KVM code before, or at least how it
depended on the enum.  But the BUILD_BUG_ON() will surely make the
ignorant among us aware _now_.  Looks good:

Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>