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[35.247.111.240]) by smtp.gmail.com with ESMTPSA id y13sm8198630pgp.16.2021.05.18.13.23.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 May 2021 13:23:45 -0700 (PDT) Date: Tue, 18 May 2021 20:23:42 +0000 From: Sean Christopherson To: Reiji Watanabe Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 06/43] KVM: x86: Properly reset MMU context at vCPU RESET/INIT Message-ID: References: <20210424004645.3950558-1-seanjc@google.com> <20210424004645.3950558-7-seanjc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 17, 2021, Reiji Watanabe wrote: > > void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event) > > { > > + unsigned long old_cr0 = kvm_read_cr0(vcpu); > > + unsigned long old_cr4 = kvm_read_cr4(vcpu); > > + > > kvm_lapic_reset(vcpu, init_event); > > > > vcpu->arch.hflags = 0; > > @@ -10483,6 +10485,10 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event) > > vcpu->arch.ia32_xss = 0; > > > > static_call(kvm_x86_vcpu_reset)(vcpu, init_event); > > + > > + if (kvm_cr0_mmu_role_changed(old_cr0, kvm_read_cr0(vcpu)) || > > + kvm_cr4_mmu_role_changed(old_cr4, kvm_read_cr4(vcpu))) > > + kvm_mmu_reset_context(vcpu); > > } > > I'm wondering if kvm_vcpu_reset() should call kvm_mmu_reset_context() > for a change in EFER.NX as well. Oooh. So there _should_ be no need. Paging has to be enabled for EFER.NX to be relevant, and INIT toggles CR0.PG 1=>0 if paging was enabled and so is guaranteed to trigger a context reset. And we do want to skip the context reset, e.g. INIT-SIPI-SIPI when the vCPU has paging disabled should continue using the same MMU. But, kvm_calc_mmu_role_common() neglects to ignore NX if CR0.PG=0, and so the MMU role will be stale if INIT clears EFER.NX without forcing a context reset. However, that's benign from a functionality perspective because the context itself correctly incorporates CR0.PG, it's only the role that's borked. I.e. KVM will fail to reuse a page/context due to the spurious role.nxe, but the permission checks are always be correct. I'll add a comment here and send a patch to fix the role calculation.