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 Andrew,

On 10/01/2018 10:53 AM, Andrew Cooper wrote:
> 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@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?
> 
> 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?

I am happy in principle. If you don't have time to work on it, I will 
try to have a look before Xen 4.12.

Cheers,

-- 
Julien Grall

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