Re: [Qemu-devel] [PATCH 05/23] hyperv: ensure VP index equal to QEMU cpu_index

[Eduardo Habkost]

On Wed, Jun 14, 2017 at 01:26:44PM +0200, Paolo Bonzini wrote:
> 
> 
> On 14/06/2017 13:25, Roman Kagan wrote:
> >> The problem with that is that it will break as soon as we create
> >> VCPUs in a different order.  Unsolvable on hosts that don't allow
> >> HV_X64_MSR_VP_INDEX to be set, however.
> > Right, thanks for putting together a detailed explanation.
> > 
> > This was a thinko back then, not to have HV_X64_MSR_VP_INDEX maintained
> > by QEMU.  I'm going to post a patch to KVM fixing that.
> > 
> > Meanwhile QEMU needs a way to maintain its notion of vp_index that is
> >   1) in sync with kernel's notion
> >   2) also with kernels that don't support setting the msr
> >   3) persistent across migrations
> > 
> > cpu_index looked like a perfect candidate.
> > 
> 
> What you want is the APIC id, which _is_ cpu_index but may not be in the
> future.  But the APIC id is also the KVM vcpu_id, so there's no need to
> have VP_INDEX maintained by QEMU.

No, KVM really uses the VCPU _index_ for HV_X64_MSR_VP_INDEX:

kvm_hv_get_msr():

        case HV_X64_MSR_VP_INDEX: {
                int r;
                struct kvm_vcpu *v;

                kvm_for_each_vcpu(r, v, vcpu->kvm) {
                        if (v == vcpu) {
                                data = r;
                                break;
                        }
                }
                break;
        }


-- 
Eduardo