linux-kernel - Re: [RFC PATCH v2 32/69] KVM: x86: Allow host-initiated WRMSR to set X2APIC regardless of CPUID

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Date:   Tue, 6 Jul 2021 16:09:32 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     isaku.yamahata@...el.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     isaku.yamahata@...il.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [RFC PATCH v2 32/69] KVM: x86: Allow host-initiated WRMSR to set
 X2APIC regardless of CPUID

On 03/07/21 00:04, isaku.yamahata@...el.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.