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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id br14sm2296525ejb.61.2021.04.21.05.03.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 21 Apr 2021 05:03:38 -0700 (PDT) Subject: Re: [PATCH 0/3] KVM: x86: guest interface for SEV live migration To: Sean Christopherson Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, srutherford@google.com, joro@8bytes.org, brijesh.singh@amd.com, thomas.lendacky@amd.com, venu.busireddy@oracle.com, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Borislav Petkov , x86@kernel.org, Ashish Kalra References: <20210420112006.741541-1-pbonzini@redhat.com> <05129de6-c8d9-de94-89e7-6257197433ef@redhat.com> <4b96c4fc-23a4-0bd2-ea58-fa6d81e50b15@redhat.com> From: Paolo Bonzini Message-ID: <478030b2-7521-4080-c3de-46533c2cc3cf@redhat.com> Date: Wed, 21 Apr 2021 14:03:37 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: 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 21/04/21 01:20, Sean Christopherson wrote: > If userspace blindly copies CPUID, but doesn't > enable the capability, the guest will think the hypercall is supported. The > guest hopefully won't freak out too much on the resulting -KVM_ENOSYS, but it > does make the CPUID flag rather useless. Yes that's why the CPUID bit must *not* be in KVM_GET_SUPPORTED_CPUID. > The > guest hopefully won't freak out too much on the resulting -KVM_ENOSYS, but it > does make the CPUID flag rather useless. > > We can make it work with: > > u64 gpa = a0, npages = a1, enc = a2; > > if (!guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS)) > break; > > if (!PAGE_ALIGNED(gpa) || !npages || > gpa_to_gfn(gpa) + npages <= gpa_to_gfn(gpa)) { > ret = -EINVAL; > break; > } > > if (!vcpu->kvm->arch.hypercall_exit_enabled) { > ret = 0; > break; > } > > [...] The interaction with KVM_CAP_ENFORCE_PV_FEATURE_CPUID scares me. But I'll take it into account when posting v2. >>>> (BTW, it's better to return a bitmask of hypercalls that will exit to >>>> userspace from KVM_CHECK_EXTENSION. Userspace can still reject with -ENOSYS >>>> those that it doesn't know, but it's important that it knows in general how >>>> to handle KVM_EXIT_HYPERCALL). > > Speaking of bitmasks, what about also accepting a bitmask for enabling the > capability? (not sure if the above implies that). E.g. Yes, makes sense. Paolo