Re: [PATCH v2 13/13] KVM: nSVM: Only copy NP_ENABLE from VMCB01's misc_ctl

Yosry Ahmed
2025-12-09T18:38:31Z

On Tue, Dec 09, 2025 at 08:23:28AM -0800, Sean Christopherson wrote:
> On Mon, Nov 10, 2025, Yosry Ahmed wrote:
> > The 'misc_ctl' field in VMCB02 is taken as-is from VMCB01. However, the
> > only bit that needs to copied is NP_ENABLE.
>
> Nit, explicitly state that all other existing bits are for SEV right away, e.g.
>
> However, the only bit that needs to copied is NP_ENABLE, as all other known
> bits in misc_ctl are related to SEV guests, and KVM doesn't support nested
> virtualization for SEV guests.

Ack.

>
> > This is a nop now because other bits are for SEV guests, which do not support
> > nested. Nonetheless, this hardens against future bugs if/when other bits are
> > set for L1 but should not be set for L2.
> >
> > Opportunistically add a comment explaining why NP_ENABLE is taken from
> > VMCB01 and not VMCB02.
> >
> > Suggested-by: Jim Mattson <jmattson@google.com>
> > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> > ---
> > arch/x86/kvm/svm/nested.c | 12 ++++++++++--
> > 1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index 503cb7f5a4c5f..4e278c1f9e6b3 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -837,8 +837,16 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
> > V_NMI_BLOCKING_MASK);
> > }
> >
> > - /* Copied from vmcb01. msrpm_base can be overwritten later. */
> > - vmcb02->control.misc_ctl = vmcb01->control.misc_ctl;
> > + /*
> > + * Copied from vmcb01. msrpm_base can be overwritten later.
> > + *
> > + * NP_ENABLE in vmcb12 is only used for consistency checks. If L1
> > + * enables NPTs, KVM shadows L1's NPTs and uses those to run L2. If L1
> > + * disables NPT, KVM runs L2 with the same NPTs used to run L1. For the
> > + * latter, L1 runs L2 with shadow page tables that translate L2 GVAs to
> > + * L1 GPAs, so the same NPTs can be used for L1 and L2.
> > + */
> > + vmcb02->control.misc_ctl = vmcb01->control.misc_ctl & SVM_MISC_CTL_NP_ENABLE;
> > vmcb02->control.iopm_base_pa = vmcb01->control.iopm_base_pa;
> > vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
> >
> > --
> > 2.51.2.1041.gc1ab5b90ca-goog
> >