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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id d6sm100976edq.37.2021.06.23.07.16.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Jun 2021 07:16:48 -0700 (PDT) Subject: Re: [PATCH 07/54] KVM: x86: Alert userspace that KVM_SET_CPUID{,2} after KVM_RUN is broken To: Sean Christopherson Cc: Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Yu Zhang , Maxim Levitsky References: <20210622175739.3610207-1-seanjc@google.com> <20210622175739.3610207-8-seanjc@google.com> From: Paolo Bonzini Message-ID: Date: Wed, 23 Jun 2021 16:16:47 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <20210622175739.3610207-8-seanjc@google.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22/06/21 19:56, Sean Christopherson wrote: > + /* > + * KVM does not correctly handle changing guest CPUID after KVM_RUN, as > + * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't > + * tracked in kvm_mmu_page_role. As a result, KVM may miss guest page > + * faults due to reusing SPs/SPTEs. Alert userspace, but otherwise > + * sweep the problem under the rug. > + * > + * KVM's horrific CPUID ABI makes the problem all but impossible to > + * solve, as correctly handling multiple vCPU models (with respect to > + * paging and physical address properties) in a single VM would require > + * tracking all relevant CPUID information in kvm_mmu_page_role. That > + * is very undesirable as it would double the memory requirements for > + * gfn_track (see struct kvm_mmu_page_role comments), and in practice > + * no sane VMM mucks with the core vCPU model on the fly. > + */ > + if (vcpu->arch.last_vmentry_cpu != -1) > + pr_warn_ratelimited("KVM: KVM_SET_CPUID{,2} after KVM_RUN may cause guest instability\n"); Let's make this even stronger and promise to break it in 5.16. Paolo