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

Sean Christopherson
2025-12-15T19:38:02Z

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@vger.kernel.org
> > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.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.