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

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Julien,

> On 22 Mar 2022, at 10:16, Julien Grall <julien@xxxxxxx> wrote:
> 
> 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.

So keeping boot core in cpupool-0 but allow Dom-0 to be in any pool.
Interesting, we will check that.

Cheers
Bertrand

> 
> Cheers,
> 
> -- 
> Julien Grall