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 <[email protected]>
> ---
> 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
>
