linux-kernel - Re: [PATCH 02/23] KVM: MMU: nested EPT cannot be used in SMM

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Date:   Wed, 9 Feb 2022 22:43:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, vkuznets@...hat.com
Subject: Re: [PATCH 02/23] KVM: MMU: nested EPT cannot be used in SMM

On Fri, Feb 04, 2022, David Matlack wrote:
> On Fri, Feb 04, 2022 at 06:56:57AM -0500, Paolo Bonzini wrote:
> > The role.base.smm flag is always zero, do not bother copying it over
> > from vcpu->arch.root_mmu.
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> Reviewed-by: David Matlack <dmatlack@...gle.com>
> 
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 3 ---
> >  1 file changed, 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 9424ae90f1ef..b0065ae3cea8 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4881,9 +4881,6 @@ kvm_calc_shadow_ept_root_page_role(struct kvm_vcpu *vcpu, bool accessed_dirty,
> >  {
> >  	union kvm_mmu_role role = {0};
> >  
> > -	/* SMM flag is inherited from root_mmu */
> > -	role.base.smm = vcpu->arch.root_mmu.mmu_role.base.smm;
> 
> nit: Retaining a comment here and/or warning here would be useful.
> 
> 	/* EPT is not allowed in SMM */

EPT _is_ allowed in SMM, KVM just doesn't support it.  Specifically, KVM doesn't
emulate Parallel SMM, a.k.a. Dual-Monitor Treatment of SMIs.  Probably worth
calling that out in the changelog.  If there's a WARN, then we don't really need
a comment as blame will get someone to the "why" if they're really curious, and
most people probably would only be confused about parallel SMM comments.

> 	WARN_ONCE_ONCE(vcpu->arch.root_mmu.mmu_role.base.smm);

+1 to a WARN, if only to provide a paper trail for git blame.  Finding when
something is purely deleted is painful.

> 
> (Although I imagine it would just get removed later in the series.)
> 
> > -
> >  	role.base.level = level;
> >  	role.base.has_4_byte_gpte = false;
> >  	role.base.direct = false;
> > -- 
> > 2.31.1
> > 
> >