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

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Message-ID: <0468715595718af34a8a3551663cffa79dd3ce2e@linux.dev>
Date: Fri, 06 Feb 2026 18:29:05 +0000
From: "Yosry Ahmed" <yosry.ahmed@...ux.dev>
To: "Sean Christopherson" <seanjc@...gle.com>, "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
Subject: Re: [PATCH v3 3/8] KVM: x86: nSVM: Set vmcb02.g_pat correctly for
 nested NPT

February 6, 2026 at 10:23 AM, "Sean Christopherson" <seanjc@...gle.com> wrote:
> 
> 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.

You mean the one that goes away after this patch: https://lore.kernel.org/kvm/20260115011312.3675857-16-yosry.ahmed@linux.dev/?

> 
>  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
> >
>