linux-kernel - Re: [PATCH] kvm/x86: ARCH_CAPABILITIES should not be advertised on AMD

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Message-ID: <CALMp9eRjDczhSirSismObZnzimxq4m+3s6Ka7OxwPj5Qj6X=BA@mail.gmail.com>
Date: Thu, 26 Jun 2025 09:08:32 -0700
From: Jim Mattson <jmattson@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Alexandre Chartre <alexandre.chartre@...cle.com>, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, pbonzini@...hat.com, xiaoyao.li@...el.com, 
	x86@...nel.org, konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH] kvm/x86: ARCH_CAPABILITIES should not be advertised on AMD

On Thu, Jun 26, 2025 at 7:02 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> I don't necessarily disagree about emulating ARCH_CAPABILITIES being pointless,
> but Paolo's point about not changing ABI for existing setups still stands.  This
> has been KVM's behavior for 6 years (since commit 0cf9135b773b ("KVM: x86: Emulate
> MSR_IA32_ARCH_CAPABILITIES on AMD hosts"); 7 years, if we go back to when KVM
> enumerated support without emulating the MSR (commit 1eaafe91a0df ("kvm: x86:
> IA32_ARCH_CAPABILITIES is always supported").

FWIW, commit 1eaafe91a0df ("kvm: x86: IA32_ARCH_CAPABILITIES is always
supported") was intended to deal with live migration issues across
Intel microarchitectures. I probably just forgot about AMD at the
time, since it wasn't on my radar. I blew it. :(