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 09/04/2018 08:53 PM, 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.

I think it is doable without too much trouble on Arm. I can have a look 
at this for Xen 4.12 once this series is merged.

> 
> As for your patch here, its a gross hack, but its probably the best
> which can be done.  I have to admit that I'm surprised/concerned that
> Xen has lasted this long with such a fundamental gap in domain creation.

I am surprised too. Hopefully your rework will make everything easier to 
use.

Cheers,

-- 
Julien Grall

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