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

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Message-ID: <aWg-9POhfPxbOZ3K@google.com>
Date: Wed, 14 Jan 2026 17:12:20 -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 Thu, Jan 15, 2026, Yosry Ahmed wrote:
> On Wed, Jan 14, 2026 at 02:07:10PM -0800, Sean Christopherson wrote:
> > On Mon, Dec 15, 2025, Yosry Ahmed wrote:
> > > 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.
> > 
> > Narrator: it wasn't fine.
> > 
> > Please resend this series.  The base-commit is garbage because your working tree
> > was polluted with non-public patches, I can't quickly figure out what your "real"
> > base was, and I don't have the bandwidth to manually work through the mess.
> > 
> > In the future, please, please don't post patches against a non-public base.  It
> > adds a lot of friction on my end, and your series are quite literally the only
> > ones I've had problems with in the last ~6 months.
> 
> Sorry this keeps happening, I honestly don't know how it happened. In my
> local repo the base commit is supposedly from your tree:
> 
> 	$ git show 58e10b63777d0aebee2cf4e6c67e1a83e7edbe0f
> 
> 	commit 58e10b63777d0aebee2cf4e6c67e1a83e7edbe0f
> 	Merge: e0c26d47def7 297631388309
> 	Author: Sean Christopherson <seanjc@...gle.com>
> 	Date:   Mon Dec 8 14:58:37 2025 +0000
> 
> 	    Merge branch 'fixes'
> 
> 	    * fixes:
> 	      KVM: nVMX: Immediately refresh APICv controls as needed on nested VM-Exit
> 	      KVM: VMX: Update SVI during runtime APICv activation
> 	      KVM: nSVM: Set exit_code_hi to -1 when synthesizing SVM_EXIT_ERR (failed VMRUN)
> 	      KVM: nSVM: Clear exit_code_hi in VMCB when synthesizing nested VM-Exits
> 	      KVM: Harden and prepare for modifying existing guest_memfd memslots
> 	      KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot
> 	      KVM: selftests: Add a CPUID testcase for KVM_SET_CPUID2 with runtime updates
> 	      KVM: x86: Apply runtime updates to current CPUID during KVM_SET_CPUID{,2}
> 	      KVM: selftests: Add missing "break" in rseq_test's param parsing
> 
> But then I cannot actually find it in your tree. Perhaps I rebased the
> baseline patches accidentally :/

Argh.  And now that I checked some of my other repositories, it looks like I have
it in literally every repo _except_ the one I use to push to kvm-x86.

Double argh.  This is my fault.  12/08 lines up with the "KVM: x86 and guest_memfd
fixes for 6.19" pull request I sent on 12/10.  So it makes sense that the only
branch merged into kvm-x86/next would be 'fixes'.  I can only assume I forgot to
tag that specific incarnation.

So, my bad, and sorry for falsely accusing you.

> Anyway, I rebased and retested on top of kvm-x86/next and will resend
> shortly.

Please do, even though I've now got this version applied locally; it'd be nice
to have a conflict-free version.

Again, my apologies.