Re: [PATCH 1/3] [not-for-unstable] xen/arm: vgic-v3: Delay the initialization of the domain information

[Julien Grall <julien.grall@arm.com>]

Hi,

On 09/29/2018 12:48 AM, Andrew Cooper wrote:
> On 29/09/18 00:45, Stefano Stabellini wrote:
>> On Sat, 29 Sep 2018, Andrew Cooper wrote:
>>> On 28/09/18 21:35, Julien Grall wrote:
>>>>
>>>> On 09/28/2018 12:11 AM, Stefano Stabellini wrote:
>>>>> On Wed, 26 Sep 2018, Julien Grall wrote:
>>>>>> Hi Stefano,
>>>>>>
>>>>>> On 09/25/2018 09:45 PM, Stefano Stabellini wrote:
>>>>>>> On Tue, 4 Sep 2018, Andrew Cooper wrote:
>>>>>>>> On 04/09/18 20:35, Julien Grall wrote:
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> On 09/04/2018 08:21 PM, Julien Grall wrote:
>>>>>>>>>> A follow-up patch will require to know the number of vCPUs when
>>>>>>>>>> initializating the vGICv3 domain structure. However this
>>>>>>>>>> information
>>>>>>>>>> is
>>>>>>>>>> not available at domain creation. This is only known once
>>>>>>>>>> XEN_DOMCTL_max_vpus is called for that domain.
>>>>>>>>>>
>>>>>>>>>> In order to get the max vCPUs around, delay the domain part of the
>>>>>>>>>> vGIC
>>>>>>>>>> v3 initialization until the first vCPU of the domain is
>>>>>>>>>> initialized.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Julien Grall <julien.grall@arm.com>
>>>>>>>>>>
>>>>>>>>>> ---
>>>>>>>>>>
>>>>>>>>>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>>>>>>>>>>
>>>>>>>>>> This is nasty but I can't find a better way for Xen 4.11 and older.
>>>>>>>>>> This
>>>>>>>>>> is not necessary for unstable as the number of vCPUs is known at
>>>>>>>>>> domain
>>>>>>>>>> creation.
>>>>>>>>>>
>>>>>>>>>> Andrew, I have CCed you to know whether you have a better idea
>>>>>>>>>> where
>>>>>>>>>> to
>>>>>>>>>> place this call on Xen 4.11 and older.
>>>>>>>>> I just noticed that d->max_vcpus is initialized after
>>>>>>>>> arch_domain_create. So without this patch on Xen 4.12, it will
>>>>>>>>> not work.
>>>>>>>>>
>>>>>>>>> This is getting nastier because arch_domain_init is the one
>>>>>>>>> initialize
>>>>>>>>> the value returned by dom0_max_vcpus. So I am not entirely sure what
>>>>>>>>> to do here.
>>>>>>>> The positioning after arch_domain_create() is unfortunate, but I
>>>>>>>> couldn’t manage better with ARM's current behaviour and Jan's
>>>>>>>> insistence
>>>>>>>> that the allocation of d->vcpu was common.  I'd prefer if the
>>>>>>>> dependency
>>>>>>>> could be broken and the allocation moved earlier.
>>>>>>>>
>>>>>>>> One option might be to have an arch_check_domainconfig() (or
>>>>>>>> similar?)
>>>>>>>> which is called very early on and can sanity check the values,
>>>>>>>> including
>>>>>>>> cross-checking the vgic and max_vcpus settings?  It could even be
>>>>>>>> responsible for mutating XEN_DOMCTL_CONFIG_GIC_NATIVE into the
>>>>>>>> correct
>>>>>>>> real value.
>>>>>>>>
>>>>>>>> As for your patch here, its a gross hack, but its probably the best
>>>>>>>> which can be done.
>>>>>>> *Sighs*
>>>>>>> If that is what we have to do, it is as ugly as hell, but that is what
>>>>>>> we'll do.
>>>>>> This is the best we can do with the current code base. I think it
>>>>>> would be
>>>>>> worth reworking the code to make it nicer. I will add it in my TODO
>>>>>> list.
>>>>>>
>>>>>>> My only suggestion to marginally improve it would be instead of:
>>>>>>>
>>>>>>>> +    if ( v->vcpu_id == 0 )
>>>>>>>> +    {
>>>>>>>> +        rc = vgic_v3_real_domain_init(d);
>>>>>>>> +        if ( rc )
>>>>>>>> +            return rc;
>>>>>>>> +    }
>>>>>>> to check on d->arch.vgic.rdist_regions instead:
>>>>>>>
>>>>>>>          if ( d->arch.vgic.rdist_regions == NULL )
>>>>>>>          {
>>>>>>>             // initialize domain
>>>>>> I would prefer to keep v->vcpu_id == 0 just in case we end up to
>>>>>> re-order the
>>>>>> allocation in the future.
>>>>> I was suggesting to check on (rdist_regions == NULL) exactly for
>>>>> potential re-ordering, in case in the future we end up calling
>>>>> vcpu_vgic_init differently and somehow vcpu_init(vcpu1) is done before
>>>>> before vcpu_init(vcpu0). Ideally we would like a way to check that
>>>>> vgic_v3_real_domain_init has been called before and I thought
>>>>> rdist_regions == NULL could do just that...
>>>> What I meant by re-ordering is we manage to allocate the
>>>> re-distributors before the vCPUs are created but still need
>>>> vgic_v3_real_domain_init for other purpose.
>>>>
>>>> But vCPU initialization is potentially other issue.
>>>>
>>>> Anyway. both way have drawbacks. Yet I still prefer checking on the
>>>> vCPU. It less likely vCPU0 will not be the first one initialized.
>>> With the exception of the idle domain, all vcpus are strictly allocated
>>> in packed ascending order.  Loads of other stuff will break if that
>>> changed, so I wouldn't worry about it.
>>>
>>> Furthermore, there is no obvious reason for this behaviour to ever change.
>> OK, let's go with Julien's patch. We need a new tag for this, something
>> like:
>>
>> Acked-but-disliked-by: Stefano Stabellini <sstabellini@kernel.org>
> 
> Do bear in mind that this patch is only for 4.11 and earlier.  I've
> already fixed staging (i.e. 4.12) when it comes to knowing d->max_vcpus :)
I thought we agreed that patch is necessary for 4.12 as d->max_vcpus is 
initialized after arch_domain_init?

I am not planning to do the rework in short term. Did you do more work 
on around domain_create recently?

Cheers,

-- 
Julien Grall

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