linux-kernel - Re: [PATCH v2 13/13] KVM: nSVM: Only copy NP_ENABLE from VMCB01's misc

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Message-ID: <7gbwdgvzv4amhdonuhsjx4nuxe6rt5gy2weuk5ctxgqunl3sxy@bmrvy6uf7h6r>
Date: Tue, 9 Dec 2025 18:38:25 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jim Mattson <jmattson@...gle.com>, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 13/13] KVM: nSVM: Only copy NP_ENABLE from VMCB01's
 misc_ctl

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@...gle.com>
> > Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.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
> >