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

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

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.

Igor