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Message-ID: <cfeb3b8b02646b073d5355495ec8842ac33aeae5.camel@intel.com>
Subject: Re: [PATCH v7 035/102] KVM: x86/mmu: Explicitly check for MMIO spte
 in fast page fault
From:   Kai Huang <kai.huang@intel.com>
To:     isaku.yamahata@intel.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     isaku.yamahata@gmail.com, Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Date:   Thu, 30 Jun 2022 23:37:15 +1200
In-Reply-To: <71e4c19d1dff8135792e6c5a17d3a483bc99875b.1656366338.git.isaku.yamahata@intel.com>
References: <cover.1656366337.git.isaku.yamahata@intel.com>
         <71e4c19d1dff8135792e6c5a17d3a483bc99875b.1656366338.git.isaku.yamahata@intel.com>
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On Mon, 2022-06-27 at 14:53 -0700, isaku.yamahata@intel.com wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>=20
> 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.

SPTE_SPECIAL_MASK has been removed in latest KVM code.  The changelog needs
update.

In fact, if I understand correctly, I don't think this changelog is correct=
:

The existing code doesn't check is_mmio_spte() because:

1) If MMIO caching is enabled, MMIO fault is always handled in
handle_mmio_page_fault() before reaching here;=20

2) If MMIO caching is disabled, is_shadow_present_pte() always returns fals=
e for
MMIO spte, and is_mmio_spte() also always return false for MMIO spte, so th=
ere's
no need check here.

"A non-present entry for MMIO spte" doesn't necessarily mean
is_shadow_present_pte() will return true for it, and there's no explanation=
 at
all that for TDX guest a MMIO spte could reach here and is_shadow_present_p=
te()
returns true for it.

If this patch is ever needed, it should come with or after the patch (patch=
es)
that handles MMIO fault for TD guest.

Hi Sean, Paolo,

Did I miss anything?

>=20
> 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.
>=20
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>=20
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 17252f39bd7c..51306b80f47c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3163,7 +3163,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, s=
truct kvm_page_fault *fault)
>  		else
>  			sptep =3D fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
> =20
> -		if (!is_shadow_present_pte(spte))
> +		if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
>  			break;
> =20
>  		sp =3D sptep_to_sp(sptep);