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

[Yuji Shimada <shimada-yxb@xxxxxxxxxxxxxxx>]

On Mon, 02 Mar 2009 11:33:41 +0000
Keir Fraser <keir.fraser@xxxxxxxxxxxxx> wrote:

> 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.

If you want to assign many devices to guest, it is one of approaches
to expand GSIs statically.  You can support hot-plug easily.

    dev 0  INTA -> GSI  16
    dev 0  INTB -> GSI  17
    dev 0  INTC -> GSI  18
    dev 0  INTD -> GSI  19
    dev 1  INTA -> GSI  20
    dev 1  INTB -> GSI  21
    dev 1  INTC -> GSI  22
    dev 1  INTD -> GSI  23
    dev 2  INTA -> GSI  24
    dev 2  INTB -> GSI  25
    dev 2  INTC -> GSI  26
    dev 2  INTD -> GSI  27
    ...
    dev 31 INTA -> GSI 140
    dev 31 INTB -> GSI 141
    dev 31 INTC -> GSI 142
    dev 31 INTD -> GSI 143

Please note that _PRT method in ACPI AML should reflect GSIs. If you expand
GSIs, it will be necessary to change the _PRT method. Please see
tools/firmware/hvmloader/acpi/dsdt.asl.

Thanks,
--
Yuji Shimada


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