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

Yosry Ahmed
2025-12-15T20:11:01Z

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@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.

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