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

[Tim Deegan <tim@xxxxxxx>]

At 09:08 +0000 on 06 Nov (1352192909), Jan Beulich wrote:
> >>> 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.

I think it would be better to allow some small number of faults per
device before disabling it rather than give dom0 carte blanche.

This check is really there to stop a mad device from hosing the system
rather than to contain a malicious OS, and a properly out-of-control
device needs to be stopped or it will livelock Xen with iommu faults.
In a uniprocessor system, dom0 might never get the chance to fix it.

Tim.

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