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Wed, 14 Aug 2024 10:30:29 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFU2BfT2NwuJa2HNPFzoLk1vrbks6dJXAa8I957nKLn8O4it7LS6EYFj9dBGc7B9aDSvDn0lA== X-Received: by 2002:a05:600c:3143:b0:429:994:41a2 with SMTP id 5b1f17b1804b1-429dd22fea5mr25643545e9.7.1723656628698; Wed, 14 Aug 2024 10:30:28 -0700 (PDT) Received: from [192.168.10.47] ([151.95.101.29]) by smtp.googlemail.com with ESMTPSA id 5b1f17b1804b1-429ded1e3f7sm26074525e9.4.2024.08.14.10.30.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 14 Aug 2024 10:30:27 -0700 (PDT) Message-ID: Date: Wed, 14 Aug 2024 19:30:26 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 09/22] KVM: x86/mmu: Try "unprotect for retry" iff there are indirect SPs To: Sean Christopherson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Peter Gonda , Michael Roth , Vishal Annapurve , Ackerly Tng References: <20240809190319.1710470-1-seanjc@google.com> <20240809190319.1710470-10-seanjc@google.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 8/9/24 21:03, Sean Christopherson wrote: > Try to unprotect shadow pages if and only if indirect_shadow_pages is non- > zero, i.e. iff there is at least one protected such shadow page. Pre- > checking indirect_shadow_pages avoids taking mmu_lock for write when the > gfn is write-protected by a third party, i.e. not for KVM shadow paging, > and in the *extremely* unlikely case that a different task has already > unprotected the last shadow page. > > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/mmu/mmu.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 09a42dc1fe5a..358294889baa 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -2736,6 +2736,9 @@ bool kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa) > gpa_t gpa = cr2_or_gpa; > bool r; > > + if (!vcpu->kvm->arch.indirect_shadow_pages) > + return false; indirect_shadow_pages is accessed without a lock, so here please add a note that, while it may be stale, a false negative will only cause KVM to skip the "unprotect and retry" optimization. (This is preexisting in reexecute_instruction() and goes away in patch 18, if I'm pre-reading that part of the series correctly). Bonus points for opportunistically adding a READ_ONCE() here and in kvm_mmu_track_write(). Paolo > if (!vcpu->arch.mmu->root_role.direct) > gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL); >