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


From: Igor Mammedov
Subject: Re: [Qemu-devel] [PATCH 05/23] hyperv: ensure VP index equal to QEMU cpu_index
Date: Wed, 14 Jun 2017 15:00:27 +0200

On Wed, 14 Jun 2017 13:26:44 +0200
Paolo Bonzini <address@hidden> 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
depending on topology cpu_index won't be the same as APIC ID/vcpu_id
/AMDs odd core count/.

> future.  But the APIC id is also the KVM vcpu_id, so there's no need to
> have VP_INDEX maintained by QEMU.
agreed it'd be better to reuse vcpu_id/apic id as interface between
qemu/kvm/guest instead of adding additional cpu_index concept in ABI

> 
> Paolo




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