Re: [PATCH v3 4/6] xen/cpupool: Create different cpupools at boot time

[Julien Grall <julien@xxxxxxx>]

Hi,

On 22/03/2022 08:47, Bertrand Marquis wrote:
> Hi Julien,
>
> > On 22 Mar 2022, at 09:35, Bertrand Marquis <bertrand.marquis@xxxxxxx> wrote:
> >
> > Hi Julien,
> >
> > > On 21 Mar 2022, at 18:44, Julien Grall <julien@xxxxxxx> wrote:
> > >
> > > Hi Bertrand,
> > >
> > > On 21/03/2022 17:19, Bertrand Marquis wrote:
> > > > > On 21 Mar 2022, at 17:36, Julien Grall <julien@xxxxxxx> wrote:
> > > > > > So I don’t know why on x86 we must have cpu0 in cpupool0, maybe x86 maintainer 
> > > > > > have more knowledge about that and
> > > > > > I can put a comment here.
> > > > > On Arm, we are not yet supporting all the CPU features that x86 supports (e.g. 
> > > > > CPU hotplug, suspend/resume...). So I a am bit concerned that the restriction 
> > > > > is just not there yet (or possibly hidden).
> > > > >
> > > > > Therefore, before lifting the restriction on Arm (and other arch), I would like 
> > > > > us to understand why it is necessary on x86.
> > > > >
> > > > > We may not have the answer quickly, so is it going to be a problem to keep the 
> > > > > restriction on Arm?
> > > > I am ok to keep the limitation to have dom0 always running on cpu0.
> > > > Only limitation I can see is that on a big little system, dom0 needs to stay on 
> > > > the type of core of the first booted core.
> > > Where does this limitation come from?
> > If dom0 must run on core0 and core0 is Little then you cannot build a system 
> > where dom0 is running on big cores.
> > If the limitation is not there, you can build such a configuration without any 
> > dependency to the boot core type.
> This might not be completely clear so let me rephrase:
> In the current system:
> - dom0 must run on cpupool-0
I don't think we need this restriction. In fact, with this series it 
will become more a problem because the cpupool ID will based on how we 
parse the Device-Tree.
So for dom0, we need to specify explicitely the cpupool to be used.

> - cpupool-0 must contain the boot core
> - consequence: dom0 must run on the boot core
>
> If boot core is little, you cannot build as system where dom0 runs only on the 
> big cores.
> Removing the second limitation (which is not required on arm) is making it 
> possible.
IMHO removing the second restriction is a lot more risky than removing 
the first one.
Cheers,

--
Julien Grall