Re: [Xen-devel] [PATCH] IOMMU: don't disable bus mastering on faults for devices used by Xen or Dom0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.11.12 at 18:15, Keir Fraser <keir@xxxxxxx> wrote:
> On 05/11/2012 16:53, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> 
>> Under the assumption that in these cases recurring faults aren't a
>> security issue and it can be expected that the drivers there are going
>> to try to take care of the problem.
>> 
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Solving an observed problem?

In the context of analyzing the situation described in
"iommu=dom0-passthrough behavior"
(http://lists.xen.org/archives/html/xen-devel/2012-11/msg00140.html)
I suppressed the IOMMU setup for some device in Dom0, and
was quite puzzled to find that only a single fault would occur.

The BM disabling is definitely a potential problem for devices Xen
itself is making use of, and I view it as wrong for Dom0 too (it
ought to be given a chance to recover - the failure mode of the
driver for the disabled device differs quite significantly before
and after this patch).

With the change at hand, a next step (depending on the outcome
of above inquiry to the IOMMU folks) might then be to insert an
artificial device to overcome the IOMMU faults. This obviously
should only be done for Xen or Dom0 used devices, i.e. could
build on top of the special casing here.

Jan


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