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

From: Igor Druzhinin <igor.druzhinin@citrix.com>
To: <paul@xen.org>, <xen-devel@lists.xenproject.org>
Cc: <wl@xen.org>, <iwj@xenproject.org>, <anthony.perard@citrix.com>,
	<andrew.cooper3@citrix.com>, <george.dunlap@citrix.com>,
	<jbeulich@suse.com>, <julien@xen.org>, <sstabellini@kernel.org>,
	<roger.pau@citrix.com>
Subject: Re: [PATCH 2/2] viridian: allow vCPU hotplug for Windows VMs
Date: Fri, 8 Jan 2021 11:35:52 +0000	[thread overview]
Message-ID: <e24a438e-3705-d035-2dd6-34fd4f558898@citrix.com> (raw)
In-Reply-To: <00a101d6e5b2$276b2140$764163c0$@xen.org>

On 08/01/2021 11:33, Paul Durrant wrote:
>> -----Original Message-----
>> From: Igor Druzhinin <igor.druzhinin@citrix.com>
>> Sent: 08 January 2021 11:30
>> To: paul@xen.org; xen-devel@lists.xenproject.org
>> Cc: wl@xen.org; iwj@xenproject.org; anthony.perard@citrix.com; andrew.cooper3@citrix.com;
>> george.dunlap@citrix.com; jbeulich@suse.com; julien@xen.org; sstabellini@kernel.org;
>> roger.pau@citrix.com
>> 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@citrix.com>
>>>> Sent: 08 January 2021 00:47
>>>> To: xen-devel@lists.xenproject.org
>>>> Cc: paul@xen.org; wl@xen.org; iwj@xenproject.org; anthony.perard@citrix.com;
>>>> andrew.cooper3@citrix.com; george.dunlap@citrix.com; jbeulich@suse.com; julien@xen.org;
>>>> sstabellini@kernel.org; roger.pau@citrix.com; Igor Druzhinin <igor.druzhinin@citrix.com>
>>>> 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@openvz.org/
>>>
>>> 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.

Igor