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

[Paolo Bonzini]

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