Re: PCI Pass-through in Xen ARM - Draft 2.

[Ian Campbell <ian.campbell@citrix.com>]

On Fri, 2015-07-31 at 09:05 +0100, Ian Campbell wrote:
> On Fri, 2015-07-31 at 13:16 +0530, Manish Jaggi wrote:
> > > > Secondly, the vdev-X entry is created async by dom0 watching on 
> > > > event.

Stefano points out that there are, confusingly, two nodes in xenstore
relating to the virtual-SBDF.

vdev-X is written by pciback and is read by pcifront, it is effectively
there to communicate the vSBDF to the guest.

vdevfn-X is written by the toolstack (libxl_create_pci_backend_device) to
tell the backend (pciback, or qemu in x86/HVM configurations using old
qemu) the vSBDF to be associated with the device.

It looks like vdevfn-X is not actually currently supported by pciback in
Linux (seemingly only the x86/HVM qemu backend consumes it). I think we
should add that support to pciback for consistency with the qemu based
backend used by x86/HVM guests.

The names are a certainly a bit confusing. We could add a new key with a
better name to communicate the vSBDF from toolstack->backend, but itseems
to me to be that would just adding even more confusion, so I recommend we
don't do that.

Once pciback supports vdevfn then libxl will be able to choose the PCI bus
layout for ARM guests in the case where the use has not requested an
explicit vdevfn for the device.

Does that make sense?

Ian.