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

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Tue, 2012-11-06 at 09:44 +0000, Tim Deegan wrote:
> > 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.
> 
Right. But moving the fault handling code to softirq should have already
helped solving/mitigating that, hasn't it?

When implementing and testing that, I wasn't able to reproduce any
livelock situation (although I can't exclude that to be at least partly
due to my inexperience, especially at the time, with I/O
virtualization)... Jan, have you (after killing the 'disable
bus-mastering part' of course)?

Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://retis.sssup.it/people/faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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