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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id jx17sm5780741ejc.60.2021.07.06.07.09.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 Jul 2021 07:09:34 -0700 (PDT) Subject: Re: [RFC PATCH v2 32/69] KVM: x86: Allow host-initiated WRMSR to set X2APIC regardless of CPUID To: isaku.yamahata@intel.com, Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H . Peter Anvin" , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , erdemaktas@google.com, Connor Kuehl , Sean Christopherson , x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: isaku.yamahata@gmail.com, Sean Christopherson References: <9b00cb86878e9986f47a0febce3c0d2872d91443.1625186503.git.isaku.yamahata@intel.com> From: Paolo Bonzini Message-ID: <21864cc4-56ed-096b-c3ff-9fc742d68624@redhat.com> Date: Tue, 6 Jul 2021 16:09:32 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <9b00cb86878e9986f47a0febce3c0d2872d91443.1625186503.git.isaku.yamahata@intel.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 03/07/21 00:04, isaku.yamahata@intel.com wrote: > Let userspace, or in the case of TDX, KVM itself, enable X2APIC even if > X2APIC is not reported as supported in the guest's CPU model. KVM > generally does not force specific ordering between ioctls(), e.g. this > forces userspace to configure CPUID before MSRs. You already have to do this, see for example MSR_IA32_PERF_CAPABILITIES: struct kvm_msr_entry msr_ent = {.index = msr, .data = 0}; if (!msr_info->host_initiated) return 1; if (guest_cpuid_has(vcpu, X86_FEATURE_PDCM) && kvm_get_msr_feature(&msr_ent)) return 1; if (data & ~msr_ent.data) return 1; Is this patch necessary? If not, I think it can be dropped. Paolo > And for TDX, vCPUs > will always run with X2APIC enabled, e.g. KVM will want/need to enable > X2APIC from time zero.