linux-kernel - Re: [PATCH] KVM: SVM: Fix redundant updates of LBR MSR intercepts

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Message-ID: <aRtVbeVHe5ZFOPQW@google.com>
Date: Mon, 17 Nov 2025 09:03:41 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: Fix redundant updates of LBR MSR intercepts

On Fri, Nov 14, 2025, Yosry Ahmed wrote:
> On Fri, Nov 14, 2025 at 08:34:54AM -0800, Sean Christopherson wrote:
> > On Wed, Nov 12, 2025, Yosry Ahmed wrote:
> > > svm_update_lbrv() always updates LBR MSRs intercepts, even when they are
> > > already set correctly. This results in force_msr_bitmap_recalc always
> > > being set to true on every nested transition,
> > 
> > Nit, it's only on VMRUN, not on every transition (i.e. not on nested #VMEXIT).
> 
> How so? svm_update_lbrv() will also be called in nested_svm_vmexit(),
> and it will eventually lead to force_msr_bitmap_recalc being set to
> true.
> 
> I guess what you meant is the "undoing the Hyper-V optimization" part.
> That is indeed only affected by the svm_update_lbrv() call in the nested
> VMRUN path.

Ooh, yeah, my mind was fully on when the intercepts would be recomputed, not on
when the flag could be set.