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

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Date:   Mon, 1 Aug 2022 15:27:11 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     isaku.yamahata@...el.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        erdemaktas@...gle.com, Sean Christopherson <seanjc@...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 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?

>  			break;
>  
>  		sp = sptep_to_sp(sptep);
> -- 
> 2.25.1
>