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Wed, 14 Aug 2024 09:40:57 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFnFK08Vl87WzQqs3F1Brre5kODaABIS4VX+/eU65GczibuUE4Dv7UmwPOShh4Rj083IwRo5w== X-Received: by 2002:a05:600c:4686:b0:428:e140:88c4 with SMTP id 5b1f17b1804b1-429dd26749dmr20611285e9.33.1723653657032; Wed, 14 Aug 2024 09:40:57 -0700 (PDT) Received: from [192.168.10.3] ([151.95.101.29]) by smtp.googlemail.com with ESMTPSA id 5b1f17b1804b1-429ded19670sm25059625e9.9.2024.08.14.09.40.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 14 Aug 2024 09:40:56 -0700 (PDT) Message-ID: <96293a7d-0347-458e-9776-d11f55894d34@redhat.com> Date: Wed, 14 Aug 2024 18:40:51 +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 03/22] KVM: x86/mmu: Trigger unprotect logic only on write-protection page faults 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> <20240809190319.1710470-4-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: > Trigger KVM's various "unprotect gfn" paths if and only if the page fault > was a write to a write-protected gfn. To do so, add a new page fault > return code, RET_PF_WRITE_PROTECTED, to explicitly and precisely track > such page faults. > > If a page fault requires emulation for any MMIO (or any reason besides > write-protection), trying to unprotect the gfn is pointless and risks > putting the vCPU into an infinite loop. E.g. KVM will put the vCPU into > an infinite loop if the vCPU manages to trigger MMIO on a page table walk. > > Fixes: 147277540bbc ("kvm: svm: Add support for additional SVM NPF error codes") > Cc: stable@vger.kernel.org Do we really want Cc: stable@ for all these patches? Most of them are of the "if it hurts, don't do it" kind; as long as there are no infinite loops in a non-killable region, I prefer not to complicate our lives with cherry picks of unknown quality. That said, this patch could be interesting for 6.11 because of the effect on prefaulting (see below). > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/mmu/mmu.c | 78 +++++++++++++++++++-------------- > arch/x86/kvm/mmu/mmu_internal.h | 3 ++ > arch/x86/kvm/mmu/mmutrace.h | 1 + > arch/x86/kvm/mmu/paging_tmpl.h | 2 +- > arch/x86/kvm/mmu/tdp_mmu.c | 6 +-- > 5 files changed, 53 insertions(+), 37 deletions(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 901be9e420a4..e3aa04c498ea 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -2914,10 +2914,8 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot, > trace_kvm_mmu_set_spte(level, gfn, sptep); > } > > - if (wrprot) { > - if (write_fault) > - ret = RET_PF_EMULATE; > - } > + if (wrprot && write_fault) > + ret = RET_PF_WRITE_PROTECTED; > > if (flush) > kvm_flush_remote_tlbs_gfn(vcpu->kvm, gfn, level); > @@ -4549,7 +4547,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault > return RET_PF_RETRY; > > if (page_fault_handle_page_track(vcpu, fault)) > - return RET_PF_EMULATE; > + return RET_PF_WRITE_PROTECTED; > > r = fast_page_fault(vcpu, fault); > if (r != RET_PF_INVALID) > @@ -4642,7 +4640,7 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu, > int r; > > if (page_fault_handle_page_track(vcpu, fault)) > - return RET_PF_EMULATE; > + return RET_PF_WRITE_PROTECTED; > > r = fast_page_fault(vcpu, fault); > if (r != RET_PF_INVALID) > @@ -4726,6 +4724,9 @@ static int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, > case RET_PF_EMULATE: > return -ENOENT; > > + case RET_PF_WRITE_PROTECTED: > + return -EPERM; Shouldn't this be a "return 0"? Even if kvm_mmu_do_page_fault() cannot fully unprotect the page, it was nevertheless prefaulted as much as possible. Paolo