linux-kernel - Re: [PATCH] KVM: VMX: Add quirk to allow L1 to set FREEZE_IN

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Message-ID: <aWbmXTJdZDO_tnvE@google.com>
Date: Tue, 13 Jan 2026 16:42:05 -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@...utronix.de>, 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>, Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: Add quirk to allow L1 to set FREEZE_IN_SMM in vmcs12

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.

> 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.

> +			return -EINVAL;
> +	}
>  
>  	if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PAT) &&
>  	    CC(!kvm_pat_valid(vmcs12->guest_ia32_pat)))
> -- 
> 2.52.0.457.g6b5491de43-goog
>