linux-kernel - Re: [PATCH 16/22] KVM: x86/mmu: remove redundant bits from extended role

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Message-ID: <Yn0XNnqnbstGSiEl@google.com>
Date:   Thu, 12 May 2022 14:18:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 16/22] KVM: x86/mmu: remove redundant bits from extended
 role

On Thu, May 12, 2022, Paolo Bonzini wrote:
> On 5/10/22 02:20, Sean Christopherson wrote:
> > --
> > From: Sean Christopherson<seanjc@...gle.com>
> > Date: Mon, 9 May 2022 17:13:39 -0700
> > Subject: [PATCH] KVM: x86/mmu: Return true from is_cr4_pae() iff CR0.PG is set
> > 
> > Condition is_cr4_pae() on is_cr0_pg() in addition to the !4-byte gPTE
> > check.  From the MMU's perspective, PAE is disabling if paging is
> > disabled.  The current code works because all callers check is_cr0_pg()
> > before invoking is_cr4_pae(), but relying on callers to maintain that
> > behavior is unnecessarily risky.
> > 
> > Fixes: faf729621c96 ("KVM: x86/mmu: remove redundant bits from extended role")
> > Signed-off-by: Sean Christopherson<seanjc@...gle.com>
> > ---
> >   arch/x86/kvm/mmu/mmu.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 909372762363..d1c20170a553 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -240,7 +240,7 @@ static inline bool is_cr0_pg(struct kvm_mmu *mmu)
> > 
> >   static inline bool is_cr4_pae(struct kvm_mmu *mmu)
> >   {
> > -        return !mmu->cpu_role.base.has_4_byte_gpte;
> > +        return is_cr0_pg(mmu) && !mmu->cpu_role.base.has_4_byte_gpte;
> >   }
> > 
> >   static struct kvm_mmu_role_regs vcpu_to_role_regs(struct kvm_vcpu *vcpu)
> 
> Hmm, thinking more about it this is not needed for two kind of opposite
> reasons:
> 
> * if is_cr4_pae() really were to represent the raw CR4.PAE value, this is
> incorrect and it should be up to the callers to check is_cr0_pg()
> 
> * if is_cr4_pae() instead represents 8-byte page table entries, then it does
> even before this patch, because of the following logic in
> kvm_calc_cpu_role():
> 
>         if (!____is_cr0_pg(regs)) {
>                 role.base.direct = 1;
>                 return role;
>         }
> 	...
>         role.base.has_4_byte_gpte = !____is_cr4_pae(regs);
> 
> 
> So whatever meaning we give to is_cr4_pae(), there is no need for the
> adjustment.

I disagree, because is_cr4_pae() doesn't represent either of those things.  It
represents the effective (not raw) CR4.PAE from the MMU's perspective.  If you
want it to represent 8-byte gPTEs, that's fine, but then please name the helper
accordingly, because is_cr4_pae() is flat out wrong if CR0.PG=0 && CR4.PAE=0.