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    SubjectRe: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN
    Date
    Paolo Bonzini <pbonzini@redhat.com> writes:

    > On 1/12/22 14:58, Vitaly Kuznetsov wrote:
    >> - best = kvm_find_cpuid_entry(vcpu, 0xD, 1);
    >> + best = cpuid_entry2_find(entries, nent, 0xD, 1);
    >> if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
    >> cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
    >> best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
    >>
    >> - best = kvm_find_kvm_cpuid_features(vcpu);
    >> + best = __kvm_find_kvm_cpuid_features(vcpu, vcpu->arch.cpuid_entries,
    >> + vcpu->arch.cpuid_nent);
    >> if (kvm_hlt_in_guest(vcpu->kvm) && best &&
    >
    > I think this should be __kvm_find_kvm_cpuid_features(vcpu, entries, nent).
    >

    Of course.

    >>
    >> + case 0x1:
    >> + /* Only initial LAPIC id is allowed to change */
    >> + if (e->eax ^ best->eax || ((e->ebx ^ best->ebx) >> 24) ||
    >> + e->ecx ^ best->ecx || e->edx ^ best->edx)
    >> + return -EINVAL;
    >> + break;
    >
    > This XOR is a bit weird. In addition the EBX test is checking the wrong
    > bits (it checks whether 31:24 change and ignores changes to 23:0).

    Indeed, however, I've tested CPU hotplug with QEMU trying different
    CPUs in random order and surprisingly othing blew up, feels like QEMU
    was smart enough to re-use the right fd)

    >
    > You can write just "(e->ebx & ~0xff000000u) != (best->ebx ~0xff000000u)".
    >
    >>
    >> + default:
    >> + if (e->eax ^ best->eax || e->ebx ^ best->ebx ||
    >> + e->ecx ^ best->ecx || e->edx ^ best->edx)
    >> + return -EINVAL;
    >
    > This one even more so.

    Thanks for the early review, I'm going to prepare a selftest and send
    this out.

    --
    Vitaly

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