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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----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?

  Paul

> Igor