linux-kernel - Re: [PATCH v3 3/8] KVM: x86: nSVM: Set vmcb02.g

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Message-ID: <aYYxh8EiLrBTiq0L@google.com>
Date: Fri, 6 Feb 2026 10:23:03 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, 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>, Shuah Khan <shuah@...nel.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH v3 3/8] KVM: x86: nSVM: Set vmcb02.g_pat correctly for
 nested NPT

On Thu, Feb 05, 2026, Jim Mattson wrote:
> When nested NPT is enabled in vmcb12, copy the (cached and validated)
> vmcb12 g_pat field to the guest PAT register. Under KVM, the guest PAT
> register lives in the vmcb02 g_pat field.
> 
> When NPT is enabled, but nested NPT is disabled, copy L1's IA32_PAT MSR to
> the vmcb02 g_pat field, since L2 shares the IA32_PAT MSR with L1.
> 
> When NPT is disabled, the vmcb02 g_pat field is ignored by hardware.

Uber nit, the "vmcb02" qualifier can be dropped, i.e.

  When NPT is disabled, the g_pat field is ignored by hardware.

Scoping it to vmcb02 makes it sound like there's a special rule about vmcb02.

> Fixes: 15038e147247 ("KVM: SVM: obey guest PAT")
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> ---
>  arch/x86/kvm/svm/nested.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 1d4ff6408b34..1ff2ede96094 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -646,9 +646,6 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
>  	struct vmcb *vmcb02 = svm->nested.vmcb02.ptr;
>  	struct kvm_vcpu *vcpu = &svm->vcpu;
>  
> -	nested_vmcb02_compute_g_pat(svm);
> -	vmcb_mark_dirty(vmcb02, VMCB_NPT);
> -
>  	/* Load the nested guest state */
>  	if (svm->nested.vmcb12_gpa != svm->nested.last_vmcb12_gpa) {
>  		new_vmcb12 = true;
> @@ -656,6 +653,19 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
>  		svm->nested.force_msr_bitmap_recalc = true;
>  	}
>  
> +	if (npt_enabled) {
> +		if (nested_npt_enabled(svm)) {
> +			if (unlikely(new_vmcb12 ||
> +				     vmcb_is_dirty(vmcb12, VMCB_NPT))) {
> +				vmcb02->save.g_pat = svm->nested.gpat;
> +				vmcb_mark_dirty(vmcb02, VMCB_NPT);
> +			}
> +		} else {
> +			vmcb02->save.g_pat = vcpu->arch.pat;
> +			vmcb_mark_dirty(vmcb02, VMCB_NPT);
> +		}
> +	}

To reduce indentation, how about this?  There's a consistency check for
nested_npt_enabled() vs. npt_enabled, so it's guaranteed to do the right thing.

	if (nested_npt_enabled(svm)) {
		if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_NPT))) {
			vmcb02->save.g_pat = svm->nested.gpat;
			vmcb_mark_dirty(vmcb02, VMCB_NPT);
		}
	} else if (npt_enabled) {
		vmcb02->save.g_pat = vcpu->arch.pat;
		vmcb_mark_dirty(vmcb02, VMCB_NPT);
	}

> +
>  	if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_SEG))) {
>  		vmcb02->save.es = vmcb12->save.es;
>  		vmcb02->save.cs = vmcb12->save.cs;
> -- 
> 2.53.0.rc2.204.g2597b5adb4-goog
>