Re: [Xen-devel] [PATCH v3 1/3] xen/arm: gic: Check the size of the CPU and vCPU interface retrieved from DT

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2015-10-06 at 15:39 +0100, Julien Grall wrote:
> > > +        csize = SZ_8K;
> > > +    }
> > > +
> > > +    /*
> > > +     * Check if the CPU interface and virtual CPU interface have the
> > > +     * same size.
> > > +     */
> > > +    if ( csize != vsize )
> > > +        printk(XENLOG_WARNING "GICv2: WARNING: "
> > > +               "Sizes of GICC (%#"PRIpaddr") and GICV (%#"PRIpaddr")
> > > don't match\n",
> > > +               csize, vsize);
> > 
> > Should we also force them to be equal? Either
> >     csize = vsize = min(csize,vsize)
> 
> If we restrict csize we will get to some other troubles later because
> vsize may be only 4KB.

Does Xen work with that? I suppose so.

> > 
> > WRT to the XXX I think I'd be happier if this was < SZ_8K for each.
> > Otherwise some future GIC which is compatible but has extensions to the
> > register space would needlessly require changes here. But I can live
> > with
> > this.
> 
> The GICv2 CPU interface is always at least 8KB. Having an higher value
> may mean that the GIC is aliased.

Or that this is a GICvN which has 8KB of GICv2 compatible registers and
then some extensions.

In either that situation or the aliasing one it would be safe to expose the
first 8KB as a gic-v2 to the guest.

> GICv2 on GICv3 is only used for guest. I prefer to restrict the usage to
> known and safe value until we have someone using different size.
> 
> This will avoid to expose unwanted data/value to a guest.

Right, I'm not saying we should expose the whole region, just the known to
be gic-v2 compatible first 8KB.

NB I'm talking about domU here, things are more complicated with dom0 and
in that case you are right that it would be a bad idea.

Ian.

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