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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.googlemail.com with ESMTPSA id i10sm17358719wmq.45.2022.01.17.03.20.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 17 Jan 2022 03:20:33 -0800 (PST) Message-ID: <517e8b95-e336-8796-6657-c0f8d554143a@redhat.com> Date: Mon, 17 Jan 2022 12:20:32 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN Content-Language: en-US To: Vitaly Kuznetsov , Sean Christopherson Cc: Igor Mammedov , kvm@vger.kernel.org, Wanpeng Li , Jim Mattson , linux-kernel@vger.kernel.org References: <16368a89-99ea-e52c-47b6-bd006933ec1f@redhat.com> <20211227183253.45a03ca2@redhat.com> <61325b2b-dc93-5db2-2d0a-dd0900d947f2@redhat.com> <87mtkdqm7m.fsf@redhat.com> <20220103104057.4dcf7948@redhat.com> <20220114095535.0f498707@redhat.com> <87ilummznd.fsf@redhat.com> <20220114122237.54fa8c91@redhat.com> <87ee5amrmh.fsf@redhat.com> <8735lmn0t1.fsf@redhat.com> From: Paolo Bonzini In-Reply-To: <8735lmn0t1.fsf@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/17/22 10:55, Vitaly Kuznetsov wrote: > No, honestly I was thinking about something much simpler: instead of > forbidding KVM_SET_CPUID{,2} after KVM_RUN completely (what we have now > in 5.16), we only forbid to change certain data which we know breaks > some assumptions in MMU, from the comment: > " > * 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. > " > It seems that CPU hotplug path doesn't need to change these so we don't > need an opt-in/opt-out, we can just forbid changing certain things for > the time being. Alternatively, we can silently ignore such changes but I > don't quite like it because it would mask bugs in VMMs. I think the version that only allows exactly the same CPUID is the best, as it leaves less room for future bugs. Paolo