Re: [PATCH V7 6/7] xl: add usb-assignable-list command

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

On Wed, 2015-10-07 at 11:55 +0200, Juergen Gross wrote:
> On 10/07/2015 10:40 AM, Ian Campbell wrote:
> > On Tue, 2015-10-06 at 17:55 +0100, George Dunlap wrote:
> > > On 25/09/15 03:11, Chunyan Liu wrote:
> > > > Add xl usb-assignable-list command to list assignable USB devices.
> > > > Assignable USB device means the USB device type is assignable and
> > > > it's not assigned to any guest yet.
> > > > 
> > > > Signed-off-by: Chunyan Liu <cyliu@suse.com>
> > > > 
> > > > ---
> > > >    Same as "libxl: add libxl_device_usb_assignable_list API" patch,
> > > >    this patch could be sqaushed to previous one. Split because of
> > > >    some dispute. Could be squashed if acceptable, otherwise could
> > > >    be removed.
> > > 
> > > I think it's worth pointing out to other reviewers that the
> > > "usb-assignable-list" command introduced here:
> > > 1. Has identical behavior to "xm usb-assignable-list", but
> > > 2. Has different behavior than "xl pci-assignable-list".
> > 
> > OOI how does xl pci-assignable-list compare to xm pci-assignable-list.
> > 
> > > Namely:
> > > 
> > > xl pci-assignable-list will list PCI devices which have been detached
> > > from their normal driver and have been assigned to pciback (in
> > > preparation for being attached to a domain).
> 
> Assigning a PCI device to pciback is a necessary step to be able to
> attach it to a domain.
> 
> > > This command will list all USB devices in dom0 that are not assigned
> > > to
> > > VMs.
> 
> Attaching a USB device to a domain works by detaching it from it's
> normal driver and attaching it to the pvUSB backend. And this backend
> is per domain (qemu), so it's not possible to prepare this step by
> attaching the device to "the" pvUSB backend.

Thanks, this is useful information which wasn't in the summary and which
gives me cause to reconsider.

> [...]
> As stated above: this will have weird consequences or you need some kind
> of placeholder backend just for this functionality. Is this really worth
> the effort?

Right. I don't think we want to invent some sort of usb placeholder backend
just for the sake of having this command work.

So it seems the question is not so much "should xl usb-assignable-list
behave like xl pci-assignable-list or not" but it is rather "what is usb
-assignable-list useful for which isn't covered by lsusb and the xl
commands for listing attached devices".

What are the intended semantics for xl usb-assignable-list wrt devices
which are currently attached to domains?

Ian.