linux-kernel - Re: [PATCH v3 02/28] KVM: x86/mmu: Check for present SPTE when clearing dirty bit in TDP MMU

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Message-ID: <Yh/Kcawg0NIjUA+5@google.com>
Date:   Wed, 2 Mar 2022 19:50:09 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3 02/28] KVM: x86/mmu: Check for present SPTE when
 clearing dirty bit in TDP MMU

On Sat, Feb 26, 2022, Sean Christopherson wrote:
> Explicitly check for present SPTEs when clearing dirty bits in the TDP
> MMU.  This isn't strictly required for correctness, as setting the dirty
> bit in a defunct SPTE will not change the SPTE from !PRESENT to PRESENT.
> However, the guarded MMU_WARN_ON() in spte_ad_need_write_protect() would
> complain if anyone actually turned on KVM's MMU debugging.
> 
> Fixes: a6a0b05da9f3 ("kvm: x86/mmu: Support dirty logging for the TDP MMU")
> Cc: Ben Gardon <bgardon@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Reviewed-by: Ben Gardon <bgardon@...gle.com>
Reviewed-by: Mingwei Zhang <mizhang@...gle.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 25148e8b711d..9357780ec28f 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1446,6 +1446,9 @@ static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>  		if (tdp_mmu_iter_cond_resched(kvm, &iter, false, true))
>  			continue;
>  
> +		if (!is_shadow_present_pte(iter.old_spte))
> +			continue;
> +
>  		if (spte_ad_need_write_protect(iter.old_spte)) {
>  			if (is_writable_pte(iter.old_spte))
>  				new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
> -- 
> 2.35.1.574.g5d30c73bfb-goog
>