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 --- arch/x86/kvm/svm/svm.c | 9 ++++++++- arch/x86/kvm/svm/svm.h | 1 + 2 files changed, 9 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 10c21e4c5406f..9d29b2e7e855d 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -705,7 +705,11 @@ void *svm_alloc_permissions_map(unsigned long size, gfp_t gfp_mask)
static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu) { - bool intercept = !(to_svm(vcpu)->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK); + struct vcpu_svm *svm = to_svm(vcpu); + bool intercept = !(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK); + + if (intercept == svm->lbr_msrs_intercepted) + return;
svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTBRANCHFROMIP, MSR_TYPE_RW, intercept); svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTBRANCHTOIP, MSR_TYPE_RW, intercept); @@ -714,6 +718,8 @@ static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
if (sev_es_guest(vcpu->kvm)) svm_set_intercept_for_msr(vcpu, MSR_IA32_DEBUGCTLMSR, MSR_TYPE_RW, intercept); + + svm->lbr_msrs_intercepted = intercept; }
void svm_vcpu_free_msrpm(void *msrpm) @@ -1221,6 +1227,7 @@ static int svm_vcpu_create(struct kvm_vcpu *vcpu) }
svm->x2avic_msrs_intercepted = true; + svm->lbr_msrs_intercepted = true;
svm->vmcb01.ptr = page_address(vmcb01_page); svm->vmcb01.pa = __sme_set(page_to_pfn(vmcb01_page) << PAGE_SHIFT); diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h index c856d8e0f95e7..dd78e64023450 100644 --- a/arch/x86/kvm/svm/svm.h +++ b/arch/x86/kvm/svm/svm.h @@ -336,6 +336,7 @@ struct vcpu_svm { bool guest_state_loaded;
bool x2avic_msrs_intercepted; + bool lbr_msrs_intercepted;
/* Guest GIF value, used when vGIF is not enabled */ bool guest_gif;
base-commit: 8a4821412cf2c1429fffa07c012dd150f2edf78c
On Wed, Nov 12, 2025, 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,
Nit, it's only on VMRUN, not on every transition (i.e. not on nested #VMEXIT).
essentially undoing the hyperv optimization in nested_svm_merge_msrpm().
When something fixes a KVM test failures (selftests or KUT), please call that out in the changelog. That way when other people encounter the failure, they'll get search hits and won't have to waste their time bisecting and/or debugging.
If you hadn't mentioned off-list that this was detected by hyperv_svm_test, I wouldn't have had the first clue as to why that test started failing. Even with the hint, it still took me a few minutes to connect the dots.
In general, be more explicit/detailed, e.g. "undoing the hyperv optimization" is unnecessarily vague, as the reader has to go look at the code to understand what you're talking about. My philosophy with changelogs is that they are write-once, read-many, and so if you can save any time/effort for readers, it's almost always worth the extra time/effort on the "write" side.
And a nit: my strong preference is to lead with what is being changed, and then dive into the details of why, what's breaking, etc. This is one of the few divergences from the tip-tree preferences. From Documentation/process/maintainer-kvm-x86.rst:
Stating what a patch does before diving into details is preferred by KVM x86 for several reasons. First and foremost, what code is actually being changed is arguably the most important information, and so that info should be easy to find. Changelogs that bury the "what's actually changing" in a one-liner after 3+ paragraphs of background make it very hard to find that information.
E.g.
-- Don't update the LBR MSR intercept bitmaps if they're already up-to-date, as unconditionally updating the intercepts forces KVM to recalculate the MSR bitmaps for vmcb02 on every nested VMRUN. Functionally, the redundant updates are benign, but forcing an update neuters the Hyper-V optimization that allows KVM to skip refreshing the vmcb12 MSR bitmap if L1 marked the "nested enlightenments" as being clean, i.e. if L1 told KVM that no changes were made to the MSR bitmap since the last VMRUN.
Clobbering the Hyper-V optimization manifests as a failure in the hyperv_svm_test KVM selftest, which intentionally changes the MSR bitmap "without telling KVM about it" to verify that KVM honors the clean hint.
==== Test Assertion Failure ==== x86/hyperv_svm_test.c:120: vmcb->control.exit_code == 0x081 pid=193558 tid=193558 errno=4 - Interrupted system call 1 0x0000000000411361: assert_on_unhandled_exception at processor.c:659 2 0x0000000000406186: _vcpu_run at kvm_util.c:1699 3 (inlined by) vcpu_run at kvm_util.c:1710 4 0x0000000000401f2a: main at hyperv_svm_test.c:175 5 0x000000000041d0d3: __libc_start_call_main at libc-start.o:? 6 0x000000000041f27c: __libc_start_main_impl at ??:? 7 0x00000000004021a0: _start at ??:? vmcb->control.exit_code == SVM_EXIT_VMMCALL
Avoid using .... --
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.
For posterity, Yosry originally proposed (off-list) fixing this by having svm_set_intercept_for_msr() check for redundant updates, but I voted against that because updating MSR interception _should_ be rare (full CPUID updates and explicit MSR filter updates), and I don't want to risk hiding a bug/flaw elsewhere. I.e. if something is triggering frequent/unexpected MSR bitmap changes, I want that to be surfaced, not squashed/handled by the low level helpers.
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
With an updated changelog,
Reviewed-by: Sean Christopherson seanjc@google.com Tested-by: Sean Christopherson seanjc@google.com
On Fri, Nov 14, 2025 at 08:34:54AM -0800, Sean Christopherson wrote:
On Wed, Nov 12, 2025, 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,
Nit, it's only on VMRUN, not on every transition (i.e. not on nested #VMEXIT).
How so? svm_update_lbrv() will also be called in nested_svm_vmexit(), and it will eventually lead to force_msr_bitmap_recalc being set to true.
I guess what you meant is the "undoing the Hyper-V optimization" part. That is indeed only affected by the svm_update_lbrv() call in the nested VMRUN path.
So we do set force_msr_bitmap_recalc on nested #VMEXIT, but it doesn't really matter because we set it again on nested VMRUN before nested_svm_merge_msrpm() is called.
essentially undoing the hyperv optimization in nested_svm_merge_msrpm().
When something fixes a KVM test failures (selftests or KUT), please call that out in the changelog. That way when other people encounter the failure, they'll get search hits and won't have to waste their time bisecting and/or debugging.
If you hadn't mentioned off-list that this was detected by hyperv_svm_test, I wouldn't have had the first clue as to why that test started failing. Even with the hint, it still took me a few minutes to connect the dots.
Noted, makes sense. I thought the fix and the original patch are in such quick succession that hopefully no one will run into it.
In general, be more explicit/detailed, e.g. "undoing the hyperv optimization" is unnecessarily vague, as the reader has to go look at the code to understand what you're talking about. My philosophy with changelogs is that they are write-once, read-many, and so if you can save any time/effort for readers, it's almost always worth the extra time/effort on the "write" side.
And a nit: my strong preference is to lead with what is being changed, and then dive into the details of why, what's breaking, etc. This is one of the few divergences from the tip-tree preferences. From Documentation/process/maintainer-kvm-x86.rst:
Stating what a patch does before diving into details is preferred by KVM x86 for several reasons. First and foremost, what code is actually being changed is arguably the most important information, and so that info should be easy to find. Changelogs that bury the "what's actually changing" in a one-liner after 3+ paragraphs of background make it very hard to find that information.
Noted.
E.g.
-- Don't update the LBR MSR intercept bitmaps if they're already up-to-date, as unconditionally updating the intercepts forces KVM to recalculate the MSR bitmaps for vmcb02 on every nested VMRUN. Functionally, the redundant updates are benign, but forcing an update neuters the Hyper-V optimization that allows KVM to skip refreshing the vmcb12 MSR bitmap if L1 marked the "nested enlightenments" as being clean, i.e. if L1 told KVM that no changes were made to the MSR bitmap since the last VMRUN.
Clobbering the Hyper-V optimization manifests as a failure in the hyperv_svm_test KVM selftest, which intentionally changes the MSR bitmap "without telling KVM about it" to verify that KVM honors the clean hint.
==== Test Assertion Failure ==== x86/hyperv_svm_test.c:120: vmcb->control.exit_code == 0x081 pid=193558 tid=193558 errno=4 - Interrupted system call 1 0x0000000000411361: assert_on_unhandled_exception at processor.c:659 2 0x0000000000406186: _vcpu_run at kvm_util.c:1699 3 (inlined by) vcpu_run at kvm_util.c:1710 4 0x0000000000401f2a: main at hyperv_svm_test.c:175 5 0x000000000041d0d3: __libc_start_call_main at libc-start.o:? 6 0x000000000041f27c: __libc_start_main_impl at ??:? 7 0x00000000004021a0: _start at ??:? vmcb->control.exit_code == SVM_EXIT_VMMCALL
Avoid using ....
Thanks!
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.
For posterity, Yosry originally proposed (off-list) fixing this by having svm_set_intercept_for_msr() check for redundant updates, but I voted against that because updating MSR interception _should_ be rare (full CPUID updates and explicit MSR filter updates), and I don't want to risk hiding a bug/flaw elsewhere. I.e. if something is triggering frequent/unexpected MSR bitmap changes, I want that to be surfaced, not squashed/handled by the low level helpers.
Hmm, that was on-list though :P
https://lore.kernel.org/kvm/aRO5ItX_--ZDfnfM@google.com/
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
With an updated changelog,
Reviewed-by: Sean Christopherson seanjc@google.com Tested-by: Sean Christopherson seanjc@google.com
Thanks!
Paolo, do you prefer a updated patch with the updated changelog, or fixing it up when you apply it?
linux-stable-mirror@lists.linaro.org
-
Sean Christopherson -
Yosry Ahmed