Re: [PATCH 09/12] xen/arm: if is_domain_direct_mapped use native addresses for GICv3

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 01/05/2020 02:31, Stefano Stabellini wrote:
> On Wed, 15 Apr 2020, Julien Grall wrote:
> > > diff --git a/xen/arch/arm/vgic-v3.c b/xen/arch/arm/vgic-v3.c
> > > index 4e60ba15cc..4cf430f865 100644
> > > --- a/xen/arch/arm/vgic-v3.c
> > > +++ b/xen/arch/arm/vgic-v3.c
> > > @@ -1677,13 +1677,25 @@ static int vgic_v3_domain_init(struct domain *d)
> >
> > I think you also want to modify vgic_v3_max_rdist_count().
> I don't think so: domUs even direct-mapped still only get 1 rdist
> region. This patch is not changing the layout of the domU gic, it is
> only finding a "hole" in the physical address space to make sure there
> are no conflicts (or at least minimize the chance of conflicts.)
How do you know the "hole" is big enough?

> > >         * Domain 0 gets the hardware address.
> > >         * Guests get the virtual platform layout.
> > This comment needs to be updated.
> Yep, I'll do
>
>
> > >         */
> > > -    if ( is_hardware_domain(d) )
> > > +    if ( is_domain_direct_mapped(d) )
> > >        {
> > >            unsigned int first_cpu = 0;
> > > +        unsigned int nr_rdist_regions;
> > >              d->arch.vgic.dbase = vgic_v3_hw.dbase;
> > >    -        for ( i = 0; i < vgic_v3_hw.nr_rdist_regions; i++ )
> > > +        if ( is_hardware_domain(d) )
> > > +        {
> > > +            nr_rdist_regions = vgic_v3_hw.nr_rdist_regions;
> > > +            d->arch.vgic.intid_bits = vgic_v3_hw.intid_bits;
> > > +        }
> > > +        else
> > > +        {
> > > +            nr_rdist_regions = 1;
> > What does promise your the rdist region will be big enough to cater all the
> > re-distributors for your domain?
> Good point. I'll add an explicit check for that with at least a warning.
> I don't think we want to return error because the configuration it is
> still likely to work.
No it is not going to work. Imagine you have have a guest with 3 vCPUs 
but the first re-distributor region can only cater 2 re-distributor. How 
is this going to be fine to continue?
For dom0, we are re-using the same hole but possibly not all of them. 
Why can't we do that for domU?
Cheers,

--
Julien Grall