Re: [Xen-devel] [PATCH v3 18/24] xen/passthrough: iommu_deassign_device_dt: By default reassign device to nobody

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.02.15 at 18:04, <ian.campbell@xxxxxxxxxx> wrote:
> On Tue, 2015-01-13 at 14:25 +0000, Julien Grall wrote:
>> Currently, when the device is deassigned from a domain, we directly reassign
>> to DOM0.
>> 
>> As the device may not have been correctly reset, this may lead to corruption 
> or
>> expose some part of DOM0 memory. Also, we may have no way to reset some
>> platform devices.
>> 
>> If Xen reassigns the device to "nobody", it may receive some global/context
>> fault because the transaction has failed (indeed the context has been
>> marked invalid). Unfortunately there is no simple way to quiesce a buggy
>> hardware. I think we could live with that for a first version of platform
>> device passthrough.
>> 
>> DOM0 will have to issue an hypercall to assign the device to itself if it
>> wants to use it.
> 
> Does this behaviour differ from x86? If so then it is worth calling that
> out explicitly (even if not, good to know I think!)

Indeed this is different from x86, and I think such behavior should
be consistent across architectures. If Dom0 isn't handed back the
device, who's going to do the supposedly prerequisite reset? On
x86/PCI that's pciback's job, and it would be illegitimate for the
driver to do _anything_ with the device if it wasn't owned by Dom0.

Jan


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