Re: [Xen-devel] [PATCH v3 22/24] tools/libxl: arm: Use an higher value for the GIC phandle

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Mon, 2015-02-23 at 22:02 +0000, Julien Grall wrote:
> 
> On 23/02/2015 14:36, Ian Campbell wrote:
> > On Thu, 2015-01-29 at 13:48 +0000, Julien Grall wrote:
> >> On 29/01/15 12:28, Stefano Stabellini wrote:
> >>> On Thu, 29 Jan 2015, Julien Grall wrote:
> >>>> On 29/01/15 11:07, Stefano Stabellini wrote:
> >>>>> On Tue, 13 Jan 2015, Julien Grall wrote:
> >>>>>> The partial device tree may contains phandle. The Device Tree Compiler
> >>>>>> tends to allocate the phandle from 1.
> >>>>>>
> >>>>>> Reserve the ID 65000 for the GIC phandle. I think we can safely assume
> >>>>>> that the partial device tree will never contain a such ID.
> >>>>>>
> >>>>>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> >>>>>> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> >>>>>> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> >>>>>>
> >>>>>
> >>>>> Shouldn't we at least check that the partial device tree doesn't contain
> >>>>> a conflicting phandle?
> >>>>
> >>>> I don't think so. This will unlikely happen, and if it happens the guest
> >>>> will crash with an obvious error.
> >>>
> >>> It is good that the error is obvious.
> >>>
> >>> But how expensive is to check for it?
> >>
> >> I would have to check the validity of the properties (name + value
> >> size). At least the properties "linux,phandle" and "phandle" should be
> >> checked.
> >>
> >> Though I could do in copy_properties but I find it hackish.
> >
> > Can't you just track the largest phandle ever seen during
> > copy_properties and then use N+1 for the GIC?
> 
> Now the we decided to trust the input device tree, it would be easier to 
> write the code.
> 
> I will give a look.
> 
> >
> >>> Think about the poor user that ends up in this situation: the fact that
> >>> is unlikely only makes it harder for a user to figure out what to do to
> >>> fix it.
> >>
> >> The poor use will have to write his device tree by hand to hit this
> >> error ;).
> >
> > Or use a new version of dtc which does things differently for some
> > reason.
> 
> And you would not be able to get a phandle for the GIC if largest 
> phandle is too high.
> 
> So the guest won't work correctly.

Indeed, filling in a bitmap as you go might be another option, although
you'd either need an 8k bitmap (not so bad in userspace) or to realloc
as it grows.

Ian.


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