linux-kernel - Re: [PATCH v4 2/3] KVM: x86: Provide a capability to disable APERF/MPERF read intercepts

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Message-ID: <CALMp9eTi_8T3Yyz6NYqqmKsgTLYKVz++9qt=2gdoxty40Od5Lw@mail.gmail.com>
Date: Tue, 24 Jun 2025 17:11:53 -0700
From: Jim Mattson <jmattson@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v4 2/3] KVM: x86: Provide a capability to disable
 APERF/MPERF read intercepts

On Tue, Jun 24, 2025 at 4:31 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Question: what do we want to do about nested?  Due to differences between SVM
> and VMX at the time you posted your patches, this series _as posted_ will do
> nested passthrough for SVM, but not VMX (before the MSR rework, SVM auto-merged
> bitmaps for all MSRs in svm_direct_access_msrs).
>
> As I've got it locally applied, neither SVM nor VMX will do passthrough to L2.
> I'm leaning toward allowing full passthrough, because (a) it's easy, (b) I can't
> think of any reason not to, and (c) SVM's semi-auto-merging logic means we could
> *unintentinally* do full passthrough in the future, in the unlikely event that
> KVM added passthrough support for an MSR in the same chunk as APERF and MPERF.

I think full passthrough makes sense.