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 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 :)

~Andrew

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