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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id a13-20020a170902eccd00b0016be596c8afsm2170943plh.282.2022.09.21.08.41.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Sep 2022 08:41:12 -0700 (PDT) Date: Wed, 21 Sep 2022 15:41:08 +0000 From: Sean Christopherson To: Vitaly Kuznetsov Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, David Matlack , Mingwei Zhang , Yan Zhao , Ben Gardon Subject: Re: [PATCH v4 1/9] KVM: x86/mmu: Bug the VM if KVM attempts to double count an NX huge page Message-ID: References: <20220830235537.4004585-1-seanjc@google.com> <20220830235537.4004585-2-seanjc@google.com> <87tu50oohn.fsf@redhat.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: kvm@vger.kernel.org On Wed, Sep 21, 2022, Sean Christopherson wrote: > On Wed, Sep 21, 2022, Vitaly Kuznetsov wrote: > > [ 962.257992] ept_fetch+0x504/0x5a0 [kvm] > > [ 962.261959] ept_page_fault+0x2d7/0x300 [kvm] > > [ 962.287701] kvm_mmu_page_fault+0x258/0x290 [kvm] > > [ 962.292451] vmx_handle_exit+0xe/0x40 [kvm_intel] > > [ 962.297173] vcpu_enter_guest+0x665/0xfc0 [kvm] > > [ 962.307580] vcpu_run+0x33/0x250 [kvm] > > [ 962.311367] kvm_arch_vcpu_ioctl_run+0xf7/0x460 [kvm] > > [ 962.316456] kvm_vcpu_ioctl+0x271/0x670 [kvm] > > [ 962.320843] __x64_sys_ioctl+0x87/0xc0 > > [ 962.324602] do_syscall_64+0x38/0x90 > > [ 962.328192] entry_SYSCALL_64_after_hwframe+0x63/0xcd > > Ugh, past me completely forgot the basics of shadow paging[*]. The shadow MMU > can reuse existing shadow pages, whereas the TDP MMU always links in new pages. > > I got turned around by the "doesn't exist" check, which only means "is there > already a _SPTE_ here", not "is there an existing SP for the target gfn+role that > can be used". > > I'll drop the series from the queue, send a new pull request, and spin a v5 > targeting 6.2, which amusing will look a lot like v1... Huh. I was expecting more churn, but dropping the offending patch and then "reworking" the series yields a very trivial overall diff. Vitaly, can you easily re-test with the below, i.e. simply delete the KVM_BUG_ON()? I'll still spin a v5, but assuming all is well I think this can go into 6.1 and not get pushed out to 6.2. diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 54ee48a87f81..e6f19e605979 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -804,7 +804,15 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp) void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp) { - if (KVM_BUG_ON(!list_empty(&sp->possible_nx_huge_page_link), kvm)) + /* + * If it's possible to replace the shadow page with an NX huge page, + * i.e. if the shadow page is the only thing currently preventing KVM + * from using a huge page, add the shadow page to the list of "to be + * zapped for NX recovery" pages. Note, the shadow page can already be + * on the list if KVM is reusing an existing shadow page, i.e. if KVM + * links a shadow page at multiple points. + */ + if (!list_empty(&sp->possible_nx_huge_page_link)) return; ++kvm->stat.nx_lpage_splits;