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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 <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, erdemaktas@google.com,
        Connor Kuehl <ckuehl@redhat.com>,
        Sean Christopherson <seanjc@google.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     isaku.yamahata@gmail.com,
        Sean Christopherson <sean.j.christopherson@intel.com>
References: <cover.1625186503.git.isaku.yamahata@intel.com>
 <9b00cb86878e9986f47a0febce3c0d2872d91443.1625186503.git.isaku.yamahata@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <21864cc4-56ed-096b-c3ff-9fc742d68624@redhat.com>
Date:   Tue, 6 Jul 2021 16:09:32 +0200
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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.