Re: Default controller type for USB devices

[Wei Liu <wei.liu2@citrix.com>]

On Fri, Jul 29, 2016 at 02:15:24PM +0100, George Dunlap wrote:
> 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?
> 

Note that I haven't been following closely the HVM/PV USB development,
but I think your analysis makes sense.

Wei.

>  -George

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