Re: [PATCH 2/2] viridian: allow vCPU hotplug for Windows VMs

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 08/01/2021 11:40, Paul Durrant wrote:
>> -----Original Message-----
>> From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
>> Sent: 08 January 2021 11:36
>> To: paul@xxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: wl@xxxxxxx; iwj@xxxxxxxxxxxxxx; anthony.perard@xxxxxxxxxx; 
>> andrew.cooper3@xxxxxxxxxx;
>> george.dunlap@xxxxxxxxxx; jbeulich@xxxxxxxx; julien@xxxxxxx; 
>> sstabellini@xxxxxxxxxx;
>> roger.pau@xxxxxxxxxx
>> Subject: Re: [PATCH 2/2] viridian: allow vCPU hotplug for Windows VMs
>>
>> On 08/01/2021 11:33, Paul Durrant wrote:
>>>> -----Original Message-----
>>>> From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
>>>> Sent: 08 January 2021 11:30
>>>> To: paul@xxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx
>>>> Cc: wl@xxxxxxx; iwj@xxxxxxxxxxxxxx; anthony.perard@xxxxxxxxxx; 
>>>> andrew.cooper3@xxxxxxxxxx;
>>>> george.dunlap@xxxxxxxxxx; jbeulich@xxxxxxxx; julien@xxxxxxx; 
>>>> sstabellini@xxxxxxxxxx;
>>>> roger.pau@xxxxxxxxxx
>>>> Subject: Re: [PATCH 2/2] viridian: allow vCPU hotplug for Windows VMs
>>>>
>>>> On 08/01/2021 08:38, Paul Durrant wrote:
>>>>>> -----Original Message-----
>>>>>> From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
>>>>>> Sent: 08 January 2021 00:47
>>>>>> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
>>>>>> Cc: paul@xxxxxxx; wl@xxxxxxx; iwj@xxxxxxxxxxxxxx; 
>>>>>> anthony.perard@xxxxxxxxxx;
>>>>>> andrew.cooper3@xxxxxxxxxx; george.dunlap@xxxxxxxxxx; jbeulich@xxxxxxxx; 
>>>>>> julien@xxxxxxx;
>>>>>> sstabellini@xxxxxxxxxx; roger.pau@xxxxxxxxxx; Igor Druzhinin 
>>>>>> <igor.druzhinin@xxxxxxxxxx>
>>>>>> Subject: [PATCH 2/2] viridian: allow vCPU hotplug for Windows VMs
>>>>>>
>>>>>> If Viridian extensions are enabled, Windows wouldn't currently allow
>>>>>> a hotplugged vCPU to be brought up dynamically. We need to expose a 
>>>>>> special
>>>>>> bit to let the guest know we allow it. It appears we can just start 
>>>>>> exposing
>>>>>> it without worrying too much about compatibility - see relevant QEMU
>>>>>> discussion here:
>>>>>>
>>>>>> https://patchwork.kernel.org/project/qemu-devel/patch/1455364815-19586-1-git-send-email-
>>>>>> den@xxxxxxxxxx/
>>>>>
>>>>> I don't think that discussion really confirmed it was safe... just that 
>>>>> empirically it appeared to
>>>> be so. I think we should err on
>>>>> the side of caution and have this behind a feature flag (but I'm happy 
>>>>> for it to default to on).
>>>>
>>>> QEMU was having this code since 2016 and nobody complained is good
>>>> enough for me - but if you insist we need an option - ok, I will add one.
>>>>
>>>
>>> Given that it has not yet been in a release, perhaps you could just guard 
>>> this and the
>> implementation of leaf 0x40000005 using HVMPV_ex_processor_masks?
>>
>> That looks sloppy and confusing to me - let's have a separate option instead.
>>
> 
> Yes, for this I guess it is really a separate thing. Using 
> HVMPV_ex_processor_masks to control the presence of leaf 0x40000005 seems 
> reasonable (since it's all about being able to use >64 vcpus). Perhaps a new 
> HVMPV_cpu_hotplug for this one?

Agree.

Igor