Re: [PATCH] xen/arm: Allow setting the number of CPUs to activate at runtime

[Michal Orzel <michal.orzel@xxxxxxx>]

Hi Julien,

On 23.05.2022 22:00, Julien Grall wrote:
> 
> 
> On 23/05/2022 11:21, Michal Orzel wrote:
>> Hi Julien,
> 
> Hi Michal,
> 
>>
>> On 23.05.2022 12:05, Julien Grall wrote:
>>> Hi,
>>>
>>> On 23/05/2022 10:13, Michal Orzel wrote:
>>>> Introduce a command line parameter "maxcpus" on Arm to allow adjusting
>>>> the number of CPUs to activate.
>>>
>>> The current definition "maxcpus" is not really suitable for big.LITTLE 
>>> systems as you have no flexibility to say how many types of each cores you 
>>> want to boot.
>>>
>>> Instead, Xen will pick-up the first CPUs it parsed from the firmware tables.
>>>
>>>
>>> So what's your use-case/target?
>>>
>> - use cases where we have no big little (although even on big.LITTLE 
>> limiting this number makes sense if we do not care about the types)
> 
> This may make sense in debug build, but for prod I think you need some 
> certainty how which CPUs you are going to use.
My conviction was that using big.LITTLE by enabling hmp-unsafe is not really 
used in the production systems (after all it's called *unsafe*)
as it may easily end up in an insecure/unstable platform without specifying the 
cpu affinity (which must be done carefully).

> 
> So I would like a warning in the documentation "maxcpus" that in big.LITTLE 
> system, there are no guarantee on which types will be used.
I'm fully ok with adding this warning.

**WARNING: On Arm big.LITTLE systems, when `hmp-unsafe` option is enabled, this 
command line
option does not guarantee on which CPU types will be used.**

> 
> This is technically a lie, but I don't want a user to start relying on how 
> Xen will parse the DT.
> 
>> - debug cases where we want to set maxcpus=1
> 
> Thanks for the clarification. I would be happy to add my tag with a warning 
> in the documentation.
> 
Does it mean you want to do this on commit or should I handle it in v2?

> Cheers,
> 

Cheers,
Michal