RE: [PATCH 0/7] hv: CPU onlining/offlining fixes and improvements

[Dexuan Cui]

> From: Stephen Hemminger [mailto:stephen@xxxxxxxxxxxxxxxxxx]
> Sent: Sunday, November 27, 2016 01:06
> To: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; KY Srinivasan
> <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; Dexuan Cui
> <decui@xxxxxxxxxxxxx>
> Subject: Re: [PATCH 0/7] hv: CPU onlining/offlining fixes and improvements
> 
> On Fri, 25 Nov 2016 13:48:36 +0100
> Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> 
> > Some time ago we forbade CPU offlining for Hyper-V and this was sufficient
> > if you boot with all CPUs onlined. Turns out, people may want to limit the
> > number online CPUs by passing 'maxcpus=' kernel parameter and we hit a
> > crash in Hyper-V code in this case. After some thinking, I think we may not
> > only fix the crash but also make the offlining prevention fine-grained: we
> > need to prevent from offlining CPUs which have VMBus channels attached
> > only. All offlined CPUs may always be onlined.
> >
> 
> As a temporary solution this is ok, but long term we need to support
> dynamic CPU online/offline.

If a CPU is bound to some channels, it seems impossible to make it offline,
unless Hyper-V supplies a mechanism to dynamically rebind  the channels (i.e.
without closing & opening the channels) to another CPU, e.g. CPU0.
Currently it looks there is no such mechanism.

For CPU online, my understanding is: this patchset of Vitaly actually makes it
work (see PATCH 6/7).

Thanks,
-- Dexuan