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

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Message-ID: <rlwgjee2tjf26jyvdwipdwejqgsira63nvn2r3zczehz3argi4@uarbt5af3wv2>
Date: Mon, 15 Dec 2025 20:10:50 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
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 Mon, Dec 15, 2025 at 11:38:00AM -0800, Sean Christopherson wrote:
> On Mon, Dec 15, 2025, Yosry Ahmed wrote:
> > On Mon, Dec 15, 2025 at 07:26:54PM +0000, 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, essentially undoing the
> > > hyperv optimization in nested_svm_merge_msrpm().
> > > 
> > > Fix it by keeping track of whether LBR MSRs are intercepted or not and
> > > only doing the update if needed, similar to x2avic_msrs_intercepted.
> > > 
> > > Avoid using svm_test_msr_bitmap_*() to check the status of the
> > > intercepts, as an arbitrary MSR will need to be chosen as a
> > > representative of all LBR MSRs, and this could theoretically break if
> > > some of the MSRs intercepts are handled differently from the rest.
> > > 
> > > Also, using svm_test_msr_bitmap_*() makes backports difficult as it was
> > > only recently introduced with no direct alternatives in older kernels.
> > > 
> > > Fixes: fbe5e5f030c2 ("KVM: nSVM: Always recalculate LBR MSR intercepts in svm_update_lbrv()")
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> > 
> > Sigh.. I had this patch file in my working directory and it was sent by
> > mistake with the series, as the cover letter nonetheless. Sorry about
> > that. Let me know if I should resend.
> 
> Eh, it's fine for now.  The important part is clarfying that this patch should
> be ignored, which you've already done.

FWIW that patch is already in Linus's tree so even if someone applies
it, it should be fine.