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

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

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.

Cheers,

-- 
Julien Grall

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