linux-kernel - Re: [RFC PATCH v6 036/104] KVM: x86/mmu: Explicitly check for MMIO spte in fast page fault

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Message-ID: <YuhheIdg47zCDiNi@google.com>
Date:   Mon, 1 Aug 2022 23:27:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Sagi Shahar <sagis@...gle.com>
Subject: Re: [RFC PATCH v6 036/104] KVM: x86/mmu: Explicitly check for MMIO
 spte in fast page fault

On Mon, Aug 01, 2022, David Matlack wrote:
> On Thu, May 05, 2022 at 11:14:30AM -0700, isaku.yamahata@...el.com wrote:
> > From: Sean Christopherson <sean.j.christopherson@...el.com>
> > 
> > Explicitly check for an MMIO spte in the fast page fault flow.  TDX will
> > use a not-present entry for MMIO sptes, which can be mistaken for an
> > access-tracked spte since both have SPTE_SPECIAL_MASK set.
> > 
> > MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so this
> > patch does not affect them.  TDX will handle MMIO emulation through a
> > hypercall instead.
> > 
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.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 d1c37295bb6e..4a12d862bbb6 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3184,7 +3184,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> >  		else
> >  			sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
> >  
> > -		if (!is_shadow_present_pte(spte))
> > +		if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> 
> I wonder if this patch is really necessary. is_shadow_present_pte()
> checks if SPTE_MMU_PRESENT_MASK is set (which is bit 11, not
> shadow_present_mask). Do TDX VMs set bit 11 in MMIO SPTEs?

This patch should be unnecessary, TDX's not-present SPTEs was one of my motivations
for adding MMU_PRESENT.   Bit 11 most definitely must not be set for MMIO SPTEs.