Re: [PATCH v2] Introduce KVM_X86_QUIRK_VMCS12_FREEZE_IN_SMM_CC

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 05, 2026, Jim Mattson wrote:
> Add KVM_X86_QUIRK_VMCS12_FREEZE_IN_SMM_CC to allow L1 to set FREEZE_IN_SMM
> in vmcs12's GUEST_IA32_DEBUGCTL field, as permitted prior to
> commit 6b1dd26544d0 ("KVM: VMX: Preserve host's DEBUGCTLMSR_FREEZE_IN_SMM
> while running the guest").  The quirk is enabled by default for backwards
> compatibility; userspace can disable it via KVM_CAP_DISABLE_QUIRKS2 for
> consistency with the constraints on WRMSR(IA32_DEBUGCTL).
> 
> Note that the quirk only bypasses the consistency check. The vmcs02 bit is
> still owned by the host, and PMCs are not frozen during virtualized SMM.
> In particular, if a host administrator decides that PMCs should not be
> frozen during physical SMM, then L1 has no say in the matter.
> 
> Fixes: 095686e6fcb4 ("KVM: nVMX: Check vmcs12->guest_ia32_debugctl on nested VM-Enter")
> Signed-off-by: Jim Mattson <jmattson@google.com>

I'll tag for stable, but otherwise LGTM.  It's too late for 6.19, but I'll get
it queued up for 6.20 after giving others a chance to react.  It'll miss the
initial batch of pull requests, but I'll try to send a fixes for the second half
of the merge window (I just realized I need to create+send the pull requests...)