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Date:   Thu, 3 Mar 2022 18:08:47 +0000
From:   Mingwei Zhang <mizhang@google.com>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        David Hildenbrand <david@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, David Matlack <dmatlack@google.com>,
        Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH v3 08/28] KVM: x86/mmu: Batch TLB flushes from TDP MMU
 for MMU notifier change_spte
Message-ID: <YiEELzrSymLY05zm@google.com>
References: <20220226001546.360188-1-seanjc@google.com>
 <20220226001546.360188-9-seanjc@google.com>
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In-Reply-To: <20220226001546.360188-9-seanjc@google.com>
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On Sat, Feb 26, 2022, Sean Christopherson wrote:
> Batch TLB flushes (with other MMUs) when handling ->change_spte()
> notifications in the TDP MMU.  The MMU notifier path in question doesn't
> allow yielding and correcty flushes before dropping mmu_lock.
nit: correctly
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 848448b65703..634a2838e117 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1203,13 +1203,12 @@ static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter,
>   */
>  bool kvm_tdp_mmu_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
> -	bool flush = kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn);
> -
> -	/* FIXME: return 'flush' instead of flushing here. */
> -	if (flush)
> -		kvm_flush_remote_tlbs_with_address(kvm, range->start, 1);
> -
> -	return false;
> +	/*
> +	 * No need to handle the remote TLB flush under RCU protection, the
> +	 * target SPTE _must_ be a leaf SPTE, i.e. cannot result in freeing a
> +	 * shadow page.  See the WARN on pfn_changed in __handle_changed_spte().
> +	 */
> +	return kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn);
>  }
>  
>  /*
> -- 
> 2.35.1.574.g5d30c73bfb-goog
>