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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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>

:-)

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