Re: [PATCH] KVM: VMX: Add quirk to allow L1 to set FREEZE_IN_SMM in vmcs12

[Sean Christopherson <seanjc@google.com>]

On Thu, Jan 22, 2026, Jim Mattson wrote:
> On Tue, Jan 13, 2026 at 7:47 PM Jim Mattson <jmattson@google.com> wrote:
> > On Tue, Jan 13, 2026 at 4:42 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > On Tue, Jan 13, 2026, Jim Mattson wrote:
> > > > Add KVM_X86_QUIRK_VMCS12_FREEZE_IN_SMM to allow L1 to set
> > > > IA32_DEBUGCTL.FREEZE_IN_SMM in vmcs12 when using nested VMX.  Prior to
> > > > commit 6b1dd26544d0 ("KVM: VMX: Preserve host's
> > > > DEBUGCTLMSR_FREEZE_IN_SMM while running the guest"), L1 could set
> > > > FREEZE_IN_SMM in vmcs12 to freeze PMCs during physical SMM coincident
> > > > with L2's execution.  The quirk is enabled by default for backwards
> > > > compatibility; userspace can disable it via KVM_CAP_DISABLE_QUIRKS2 if
> > > > consistency with WRMSR(IA32_DEBUGCTL) is desired.
> > >
> > > It's probably worth calling out that KVM will still drop FREEZE_IN_SMM in vmcs02
> > >
> > >         if (vmx->nested.nested_run_pending &&
> > >             (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS)) {
> > >                 kvm_set_dr(vcpu, 7, vmcs12->guest_dr7);
> > >                 vmx_guest_debugctl_write(vcpu, vmcs12->guest_ia32_debugctl &
> > >                                                vmx_get_supported_debugctl(vcpu, false)); <====
> > >         } else {
> > >                 kvm_set_dr(vcpu, 7, vcpu->arch.dr7);
> > >                 vmx_guest_debugctl_write(vcpu, vmx->nested.pre_vmenter_debugctl);
> > >         }
> > >
> > > both from a correctness standpoint and so that users aren't mislead into thinking
> > > the quirk lets L1 control of FREEZE_IN_SMM while running L2.
> >
> > Yes, it's probably worth pointing out that the VM is now subject to
> > the whims of the L0 administrators.
> >
> > While that makes some sense for the legacy vPMU, where KVM is just
> > another client of host perf, perhaps the decision should be revisited
> > in the case of the MPT vPMU, where KVM owns the PMU while the vCPU is
> > in VMX non-root operation.

Eh, running guests with FREEZE_IN_SMM=0 seems absolutely crazy from a security
perspective.  If an admin wants to disable FREEZE_IN_SMM, they get to keep the
pieces.  And KVM definitely isn't going to override the admin, e.g. to allow the
guest to profile host SMM.

> > > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > > > index 0521b55d47a5..bc8f0b3aa70b 100644
> > > > --- a/arch/x86/kvm/vmx/nested.c
> > > > +++ b/arch/x86/kvm/vmx/nested.c
> > > > @@ -3298,10 +3298,24 @@ static int nested_vmx_check_guest_state(struct kvm_vcpu *vcpu,
> > > >       if (CC(vmcs12->guest_cr4 & X86_CR4_CET && !(vmcs12->guest_cr0 & X86_CR0_WP)))
> > > >               return -EINVAL;
> > > >
> > > > -     if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS) &&
> > > > -         (CC(!kvm_dr7_valid(vmcs12->guest_dr7)) ||
> > > > -          CC(!vmx_is_valid_debugctl(vcpu, vmcs12->guest_ia32_debugctl, false))))
> > > > -             return -EINVAL;
> > > > +     if (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS) {
> > > > +             u64 debugctl = vmcs12->guest_ia32_debugctl;
> > > > +
> > > > +             /*
> > > > +              * FREEZE_IN_SMM is not virtualized, but allow L1 to set it in
> > > > +              * L2's DEBUGCTL under a quirk for backwards compatibility.
> > > > +              * Prior to KVM taking ownership of the bit to ensure PMCs are
> > > > +              * frozen during physical SMM, L1 could set FREEZE_IN_SMM in
> > > > +              * vmcs12 to freeze PMCs during physical SMM coincident with
> > > > +              * L2's execution.
> > > > +              */
> > > > +             if (kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_VMCS12_FREEZE_IN_SMM))
> > > > +                     debugctl &= ~DEBUGCTLMSR_FREEZE_IN_SMM;
> > > > +
> > > > +             if (CC(!kvm_dr7_valid(vmcs12->guest_dr7)) ||
> > > > +                 CC(!vmx_is_valid_debugctl(vcpu, debugctl, false)))
> > >
> > > I'm mildly tempted to say we should quirk the entire consistency check instead of
> > > limiting it to FREEZE_IN_SMM, purely so that we don't have to add yet another quirk
> > > if a different setup breaks on a different bit.  I suppose we could limit the quirk
> > > to bits that could have been plausibly set in hardware, because otherwise VM-Entry
> > > using L2 would VM-Fail, but that's still quite a few bits.
> > >
> > > I'm definitely not opposed to a targeted quirk though.
> >
> > I have no preference.

After mulling over the options from time to time, I think our best be is to quirk
only FREEZE_IN_SMM, but very explicity scope the quirk to just the consistency
check.  E.g. maybe KVM_X86_QUIRK_VMCS12_FREEZE_IN_SMM_CC?  That should help alert
readers to the fact that the quirk bypasses the check, but L2 will still see
FREEZE_IN_SMM=0 (e.g. in the unlikely scenario L1 disables interception of
DEBUGCTL).

As for why just FREEZE_IN_SMM, in addition to the fact that FREEZE_IN_SMM is the
only bit that broke anyone (as far as we know, /knock wood), it's also the only
bit that is host-owned.  I.e. unless the host admin likes SMM mucking with things,
skipping the consistency check isn't terrible from a functionality perspective
(KVM doesn't honor the bit for emulated SMM, but that's QEMU's problem :-D).

> Would you like me to post a v2?

Yes please.