Re: [Xen-devel] [rfc 00/18] ioemu: use devfn instead of slots as the unit for passthrough

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 02/03/2009 11:25, "Simon Horman" <horms@xxxxxxxxxxxx> wrote:

>> I mean that xend can pick a virtual devfn for the device that it knows has a
>> non-conflicting GSI. This avoids any need for dynamic mapping between devfn
>> and GSI (which would be more of a pain in the neck -- for example, your
>> patch doesn't work because certain parts of BIOS info tables need to be
>> dynamically generated, as currently they hardcode the devfn-GSI
>> relationship).
> 
> Thanks for the clarification. I suspect that scheme could easily run into
> allocation problems when multi-function devices are passed-through as
> multi-function devices.  Especially in the case of hot-plug. Buy which I
> mean, it might be hard to find a device with the GSI for INTA + one or more
> of INTB, C and D are free. But I'll take a look into it and see how it
> goes.

Well, it depends how many devices you want to pass through. I bet you're
good up to at least half dozen, and likely more.

> In any case, could you be more specific about which areas my approach break?

Mainly, virtual firmware and (possibly) save/restore. The latter depends on
whether a guest with dynamically assigned devfn<->GSI relationship is ever
allowed to be saved/restored.

Your approach is also no good for PCI hotplug, since I'm pretty sure you
cannot update GSI bindings after the guest has booted. Unless there's some
ACPI magic that could be employed in this case.

 -- Keir



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