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

[Paolo Bonzini <pbonzini@redhat.com>]

On 15/06/2017 14:41, Roman Kagan wrote:
> On Wed, Jun 14, 2017 at 03:00:27PM +0200, Igor Mammedov wrote:
>> On Wed, 14 Jun 2017 13:26:44 +0200
>> Paolo Bonzini <pbonzini@redhat.com> 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/.
> 
> So vcpu_id can be sparse?

Yes.

> Having consulted the spec, I'm not so confident any more this is the
> right move.
> 
>> 7.8.1 Virtual Processor Index
>>
>> Virtual processors are identified by using an index (VP index). The
>> maximum number of virtual processors per partition supported by the
>> current implementation of the hypervisor can be obtained through CPUID
>> leaf 0x40000005. A virtual processor index must be less than the
>> maximum number of virtual processors per partition.
> 
> This seems to imply that VP index should be dense.  As if they use it
> directly as an index into an array whose length is equal to the max
> number of vcpus.  (BTW the value we report for it is currently hardcoded
> to 0x40 which probably needs fixing, too.)
> 
> I'm starting to drift back to adding SET_MSRS support to
> HV_X64_MSR_VP_INDEX in KVM to delegate the control to QEMU, and having
> QEMU use cpu_index as its value...  :-/

Yes, definitely.

Paolo