Re: [PATCH v3 2/9] xen/arm: add cache coloring initialization for domains

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.11.2022 10:41, Jan Beulich wrote:
> On 22.11.2022 21:25, Julien Grall wrote:
>> On 21/11/2022 16:40, Jan Beulich wrote:
>>> On 21.11.2022 17:23, Carlo Nonato wrote:
>>>> This would be really good. The only problem I can see is the p2m allocation
>>>> which is done during domain creation. With the current set of patches it
>>>> results in a "Failed to allocate P2M pages" since we want to have p2m 
>>>> tables
>>>> allocated with the same color of the domain and a null page is returned
>>>> because we have no colors.
>>>
>>> Hmm, I see. It would seem to me that this p2m init really is happening
>>> too early. Ideally domain_create would merely mean creating a largely
>>> empty container, with stuff being populated subsequently as necessary.
>>
>> The vGIC is not optional. So to me it sounds wrong to defer the decision 
>> to after the domain is created.
>>
>> It is not clear to me how you would check that mandatory components have 
>> been properly initialized.
> 
> There could be final checking right before unpausing a domain for the
> first time.
> 
>>> But I guess this is too much of a re-work to be done in the context
>>> here, plus of course I may be overlooking something which actually
>>> makes it necessary for domain creation to be structured the way it is
>>> right now. (Imo the reason for the early minimal population of the p2m,
>>> added only quite recently, wasn't a good one, and the vGIC init would
>>> better be deferred. Yet once again I may lack details on why that's not
>>> possible.)
>>
>> See above for the theoritical part. For the practice part, we need to 
>> know the vGIC version at domain creation because it impact the maximum 
>> of vCPU we can expose.
> 
> Sure - I didn't suggest to leave that information out of domain_create.
> 
>> It is also not very clear where this could be initialized. Are you 
>> suggesting to add an extra mandatory hypercall? FAOD, I don't think 
>> p2m_set_allocation() would be the right place.
> 
> I agree with the latter - that would at best be a bodge. And yes, I was
> thinking of having a separate domctl for that purpose.

The expand further: I think setvnumainfo would also better be issued
before _any_ memory allocations for a domain. And I view vNUMA data as
kind of comparable to the coloring data here.

Jan