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

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Message-ID: <6cozacewv4sop77ilrqnervzpifinxki2ykef55awan2ka5jdf@sqyj7jed3qii>
Date: Thu, 15 Jan 2026 00:35:33 +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 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 :/

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