linux-kernel - Re: [PATCH v7 035/102] KVM: x86/mmu: Explicitly check for MMIO spte in fast page fault

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Message-ID: <20220713083515.GQ1379820@ls.amr.corp.intel.com>
Date:   Wed, 13 Jul 2022 01:35:15 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v7 035/102] KVM: x86/mmu: Explicitly check for MMIO spte
 in fast page fault

On Thu, Jun 30, 2022 at 11:37:15PM +1200,
Kai Huang <kai.huang@...el.com> wrote:

> On Mon, 2022-06-27 at 14:53 -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.
> 
> SPTE_SPECIAL_MASK has been removed in latest KVM code.  The changelog needs
> update.

It was renamed to SPTE_TDP_AD_MASK. not removed.


> 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; 
>
> 2) If MMIO caching is disabled, is_shadow_present_pte() always returns false for
> MMIO spte, and is_mmio_spte() also always return false for MMIO spte, so there'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_pte()
> returns true for it.

Although it was needed, I noticed the following commit made this patch
unnecessary.  So I'll drop this patch. Kudos to Sean.

edea7c4fc215c7ee1cc98363b016ad505cbac9f7
"KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs"

-- 
Isaku Yamahata <isaku.yamahata@...il.com>