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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/10/18 10:43, Julien Grall wrote:
> 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@xxxxxxx>
>>>>>>>>>>>
>>>>>>>>>>> ---
>>>>>>>>>>>
>>>>>>>>>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>>>>>>>>>
>>>>>>>>>>> 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@xxxxxxxxxx>
>>
>> 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?

Oh right.

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

There are multiple related patch series out on xen-devel atm, but I
expect I need to spin a new version of each of them.  I'll see if I have
some time to put towards it.  Are you happy in principle with the
arch_check_domainconfig() plan?

~Andrew

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