linux-kernel - Re: [PATCH v2 2/3] KVM: x86/mmu: Avoid collision with !PRESENT SPTEs in TDP MMU lpage stats

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Date:   Thu, 29 Jul 2021 18:34:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 2/3] KVM: x86/mmu: Avoid collision with !PRESENT SPTEs
 in TDP MMU lpage stats

On Mon, Jul 26, 2021, Mingwei Zhang wrote:
> Factor in whether or not the old/new SPTEs are shadow-present when
> adjusting the large page stats in the TDP MMU. A modified MMIO SPTE can
> toggle the page size bit, as bit 7 is used to store the MMIO generation,
> i.e. is_large_pte() can get a false positive when called on a MMIO SPTE.
> Ditto for nuking SPTEs with REMOVED_SPTE, which sets bit 7 in its magic
> value.
> 
> Opportunistically move the logic below the check to verify at least one
> of the old/new SPTEs is shadow present.
> 
> Use is/was_leaf even though is/was_present would suffice.  The code
> generation is roughly equivalent since all flags need to be computed
> prior to the code in question, and using the *_leaf flags will minimize
> the diff in a future enhancement to account all pages, i.e. will change
> the check to "is_leaf != was_leaf".
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>

There's no hard rule for when to use Suggested-by vs. giving Author credit, but
in this case, since you took the patch and changelog verbatim[*] (sans the missing
tags below), it's more polite to take the full patch (with me as Author in
this case) and add your SOB since you're posting the patch.

  Fixes: 1699f65c8b65 ("kvm/x86: Fix 'lpages' kvm stat for TDM MMU")
  Cc: stable@...r.kernel.org

[*] https://lkml.kernel.org/r/YPho0ME5pSjqRSoc@google.com

> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>