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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

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@xxxxxxxx>
> > 
> > ---
> >   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).
> 
> This command will list all USB devices in dom0 that are not assigned to
> VMs.
> 
> Juergen and I had a long back-and-forth about it around v3.  I thought
> having slightly different semantics might be confusing, and Juergen
> thought the functionality was important to include.  We didn't really
> come to a conclusion and none of the tools maintainers expressed an
> opinion.

TBH I couldn't follow precisely what that discussion was about, so thanks
for your summary. IMHO you both make good points.

However given that xend was now removed 2 releases ago I think the time for
strictly mimicking xm behaviour purely for the sake of that
compatibility/transition has passed.

Obviously if the xm interface was fine we shouldn't deviate from it just to
be contrary, but similarly if the xm interface was bad in some way or
doesn't fit in with the direction xl has taken since the two coexisted then
we shouldn't feel bound to follow xm.

In this case and at this point in time I think I find the argument that xl
subcommands should, where possible, behave similarly to each other more
compelling than they should match their xm counterparts. (maybe if we'd
been aware of it we would have implemented pci-assignable-list differently,
but that ship has sailed).

So IMHO xl usb-assignable-list should behave like pci-assignable-list by
default.

Now, maybe it should also support some sort of --all or --full or --host
option which lists everything, ideally with some indication as to whether
they are attached to usbback or not and using syntax which can just be cut
-and-pasted into a cfg file (without at least one of those it's just a
pointless reimplementation of lsusb).

However I think --all/full/host is an optional extra.

/me wonders if xl ought to adjust it's behaviour in various ways when
called with $0==xm.

/me beats himself with sticks.

Ian.

> 

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