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Re: [Xen-devel] [PATCH 04/10] xen/arm: vgic-v3: Don't check the size when we ignore the write/read as zero

On 21/01/15 12:11, Ian Campbell wrote:
> I thought about this overnight, and I would like to keep UNPREDICATABLE
> as the current log + crash please. Apart from the fact that I don't want
> guests to be able to rely on unpredictable accesses returning 0 it is
> also more consistent with the ARM ARM which says "UNPREDICTABLE
> behaviour must not be documented or promoted as having a defined
> effect".

I was actually planning to suggest this as the access for each registers
are clearly define in the specs.

> So, in summary:
> 1) Any access which is described as UNPREDICTABLE in GIC spec 5.1.3
> should result in the current bad_width behaviour, that is: a log message
> and a domain crash.


> 2) Accesses which are valid and which are correctly emulated according
> to the features which our virtual GIC exposes to the guest should
> succeed silently, regardless of whether that means WI, read a constant
> or actually have some effect.


> 3) Accesses which are valid but which do not correctly emulate according
> to the features of the virtual gic which we are exposing can log if we
> think it is useful to do so.

I gave a look to the code. We have few registers we don't correctly
emulate. The current behavior seems to be inconsistent, we either inject
a data abort (such as ICPENDR) or ignore the error (such as ICACTIVER).

Shall we take a domain_crash approach (as bad_width) or inject a data abort?


Julien Grall

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