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Subject: Re: [RFC PATCH v2 45/69] KVM: x86/mmu: Return old SPTE from
 mmu_spte_clear_track_bits()
To:     isaku.yamahata@intel.com, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, erdemaktas@google.com,
        Connor Kuehl <ckuehl@redhat.com>,
        Sean Christopherson <seanjc@google.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     isaku.yamahata@gmail.com,
        Sean Christopherson <sean.j.christopherson@intel.com>
References: <cover.1625186503.git.isaku.yamahata@intel.com>
 <b16bac1fd1357aaf39e425aab2177d3f89ee8318.1625186503.git.isaku.yamahata@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <3ca5f199-3fd8-34b0-14e4-2d9259b6fb6e@redhat.com>
Date:   Tue, 6 Jul 2021 16:56:57 +0200
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On 03/07/21 00:04, isaku.yamahata@intel.com wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
> 
> Return the old SPTE when clearing a SPTE and push the "old SPTE present"
> check to the caller.  Private shadow page support will use the old SPTE
> in rmap_remove() to determine whether or not there is a linked private
> shadow page.
> 
> 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 | 12 +++++++-----
>   1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 0259781cee6a..6b0c8c84aabe 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -542,9 +542,9 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
>    * Rules for using mmu_spte_clear_track_bits:
>    * It sets the sptep from present to nonpresent, and track the
>    * state bits, it is used to clear the last level sptep.
> - * Returns non-zero if the PTE was previously valid.
> + * Returns the old PTE.
>    */
> -static int mmu_spte_clear_track_bits(u64 *sptep)
> +static u64 mmu_spte_clear_track_bits(u64 *sptep)
>   {
>   	kvm_pfn_t pfn;
>   	u64 old_spte = *sptep;
> @@ -555,7 +555,7 @@ static int mmu_spte_clear_track_bits(u64 *sptep)
>   		old_spte = __update_clear_spte_slow(sptep, shadow_init_value);
>   
>   	if (!is_shadow_present_pte(old_spte))
> -		return 0;
> +		return old_spte;
>   
>   	pfn = spte_to_pfn(old_spte);
>   
> @@ -572,7 +572,7 @@ static int mmu_spte_clear_track_bits(u64 *sptep)
>   	if (is_dirty_spte(old_spte))
>   		kvm_set_pfn_dirty(pfn);
>   
> -	return 1;
> +	return old_spte;
>   }
>   
>   /*
> @@ -1104,7 +1104,9 @@ static u64 *rmap_get_next(struct rmap_iterator *iter)
>   
>   static void drop_spte(struct kvm *kvm, u64 *sptep)
>   {
> -	if (mmu_spte_clear_track_bits(sptep))
> +	u64 old_spte = mmu_spte_clear_track_bits(sptep);
> +
> +	if (is_shadow_present_pte(old_spte))
>   		rmap_remove(kvm, sptep);
>   }
>   
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>