Re: [PATCH RFC V2 3/5] libxl: add pvusb API

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

On Mon, 2015-03-09 at 10:17 +0000, George Dunlap wrote:
> On 03/09/2015 09:39 AM, Ian Campbell wrote:
> > On Fri, 2015-03-06 at 16:50 +0000, George Dunlap wrote:
> >>> +libxl_device_usb *
> >>> +libxl_device_usb_assignable_list(libxl_ctx *ctx, int *num)
> >>
> >> I'm a bit ambivalent about this one.  For people using xl, "lsusb"
> >> should be just fine.  Is anyone building a toolstack on top of libxl
> >> directly going to need this functionality?  Would libvirt use this
> >> functionality, for instance, or would it use libusb?  Or just leave
> >> that to the caller?
> > 
> > Do USB devices not need to be bound to some sort of placeholder driver
> > (a la xen-pciback) or at least unbound from their actual driver before
> > they can be assigned to a guest?
> > 
> > If so then I would expect a function such as this to list the subset of
> > devices which have been correctly configured for USB passthrough, which
> > would be a subset of the output of lsusb I think.
> 
> Are you saying you think this is what the function *should* do, or are
> you saying that you think it should be renamed because that's what the
> name would lead you to expect it did?

I was saying that's what I would have expected, but also asking how USB
passthru worked because I didn't know.

> This code, following xend's lead, seems to do the plugging stuff
> automatically; similar to the "seize=1" flag I added for PCI devices.  I
> think this is probably the best interface here; the reason it's not
> enabled by default in the PCI case is that it's so dangerous (you might
> yank out your own hard disk by accident).
> 
> Given this "auto-seize" is what xend did, and what qemu does (and what
> VirtualBox does, and what presumably every other virtualization-related
> software does), I think adding the "make assignable" step would be
> unnecessary and confusing.

I agree, given non-optional auto-seize I'm not sure having a function to
enumerate the USB devices is all that useful, unless we happen to need
it internally and it is convenient to expose it.

However, it might be the sort of thing we could put in libxlutil, since
it is the sort of thing every toolstack would have to write for itself
otherwise.

Ian.