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SubjectRe: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN
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On Thu, 2022-01-13 at 10:27 +0100, Vitaly Kuznetsov wrote:
> 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.
>
I also looked at this recently (due to other reasons) and I found out that
qemu picks a parked vcpu by its vcpu_id which is its initial apic id,
thus apic id related features should not change.

Take a look at 'kvm_get_vcpu' in qemu source.
Maybe old qemu versions didn't do this?

Best regards,
Thanks,
Maxim Levitsky

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