linux-kernel - Re: [PATCH v2] Introduce KVM_X86_QUIRK_VMCS12_FREEZE_IN_SMM

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Message-ID: <aYUmjpXZfDhDiuuk@google.com>
Date: Thu, 5 Feb 2026 15:23:58 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
	Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Josh Hilke <jrhilke@...gle.com>
Subject: Re: [PATCH v2] Introduce KVM_X86_QUIRK_VMCS12_FREEZE_IN_SMM_CC

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@...gle.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...)