Re: [Xen-devel] [PATCH v5 3/5] libxl: add support for vscsi

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, May 13, 2015 at 4:12 PM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> If that is not an option then we may need to follow the pvusb model
> (Chunyan and George CC-d in case I've got it wrong) which is to have
> explicit/separate host and device structs in the libxl API and
> associated separate commands to add/remove buses vs add/remove devices
> on those buses.

I seem to recall that the end goal for pvusb was that you *could*
specify busses if you wanted to, but that if you didn't, libxl would
just do something sensible behind the scenes (as you are suggesting
with pvscsi).

One of the reasons for making this explicit was that libvirt (as I
understand it) wants to be able to make it explicit.  Another reason
is that in the USB case we're actually trying to share two different
pathways -- pvusb and dm-usb (i.e., a device passed through via qemu),
each of which are only available on some guests.  In the case of a PV
guest, it's pretty clear which one to use; but in the case of an HVM
guest, it may not be clear to the toolstack what pathway the guest OS
is capable of using.

 -George

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