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 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...

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