linux-kernel - Re: [PATCH v3 1/2] x86: KVM: Advertise FSRS and FSRC on AMD to userspace

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Message-ID: <20250411144422.GFZ_kqxnqO65es2xPs@fat_crate.local>
Date: Fri, 11 Apr 2025 16:44:22 +0200
From: Borislav Petkov <bp@...en8.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Maksim Davydov <davydov-max@...dex-team.ru>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, x86@...nel.org, babu.moger@....com,
	mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
	hpa@...or.com, jmattson@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v3 1/2] x86: KVM: Advertise FSRS and FSRC on AMD to
 userspace

On Fri, Apr 11, 2025 at 06:49:54AM -0700, Sean Christopherson wrote:
> KVM should still explicitly advertise support for AMD's flavor.  There are KVM
> use cases where KVM's advertised CPUID support is used almost verbatim, in which

So that means the vendor differentiation is done by the user and KVM shouldn't
even try to unify things...?

> case not advertising AMD_FSRC would result it the features not be enumerated to
> the guest.  Linux might cross-pollinate, but KVM can't rely on all software to do
> so.

Sure.

The AMD feature flags are called the same:

  CPUID_Fn80000021_EAX [Extended Feature 2 EAX] (Core::X86::Cpuid::FeatureExt2Eax)
  
  11 FSRC. Read-only. Reset: Fixed,1. Fast Short Repe Cmpsb supported.
  10 FSRS. Read-only. Reset: Fixed,1. Fast Short Rep Stosb supported.

just the CPUID leaf is different.

But I guess KVM isn't exporting CPUID *names* but CPUID *leafs* so the
internal naming doesn't matter.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette