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>]

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.
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel