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

[Paolo Bonzini]

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.

Paolo