Re: [Xen-devel] Default controller type for USB devices

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 29/07/16 14:00, Juergen Gross wrote:
> When specifying no USB controller type for a usb device the default is
> chosen in libxl__device_usbctrl_setdefault(). For a HVM guest this is
> currently the not yet supported "LIBXL_USBCTRL_TYPE_DEVICEMODEL".
> 
> Wouldn't it make sense to handle HVM guests in the same way as PV guests
> as long as emulated USB devices are not implemented in libxl? Or would
> this create future incompatibilities which we don't want to run into?
> 
> I'd be happy to send a patch if this is the way to go.

I think the big thing is that we can pretty much expect a typical HVM
guest OS to have drivers for the emulated hardware; we *cannot* expect
an average HVM guest OS to have pvfront drivers.  This will probably
always apply to Windows, but at the moment it even applies to Linux.

So what happens right now if a user simply asks for a usbctrl for an HVM
guest?  They get an error on domain creation.  This will hopefully
prompt them to look into either switching back to the old format, or
finding out about pvusb, at which point they can maybe find a pvusbfront
for their OS.

If we make the switch you propose, then the user will create a usb
controller, which will succeed; but if the guest OS doesn't have
pvusbfront drivers (which is likely), then they will mysteriously just
not see the controller and devices they've plugged in.

As a user, I would personally rather have an error message up front
which gives me a clue as to what's wrong, rather than have things
mysteriously not work and spend a lot of time going around trying to
figure out what's wrong.

So I think the current default is the best.  Wei / Ian, any thoughts?

 -George

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