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

[Julien Grall <julien.grall@xxxxxxx>]

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

--
Julien Grall

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