Re: [Xen-devel] [PATCH v7 RFC 1/2] libxl: Introduce functions to add and remove USB devices to an HVM guest

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 06/18/2014 02:49 PM, Ian Campbell wrote:
> > > > +        ("backend_domid",    libxl_domid),
> > > > +        ("backend_domname",  string),
> > > > +        ("u", KeyedUnion(None, libxl_device_usb_type, "type",
> > > > +                         [("hostdev", Struct(None, [
> > > > +                                ("hostbus",   integer),
> > > > +                                ("hostaddr",  integer) ]))
> > > No need to express the host topology I think (because you can build that
> > > from the bus,addr tuples)?
> > I don't really follow.  You mean, we can drop 'host' from the last two
> > elements, and just call them "bus" and "addr"?
> Gah, I started writing one thing and then reunderstood usb and wrote
> half another.
>
> What I was trying to say is that you don't need hostaddr to describe the
> full USB topology path to the device because the (bus,addr) tuple you've
> given already does so (because each hub effectively creates a new bus
> number, so they all look like toplevel buses in this representation).
You seem to be saying that something is redundant, or that there's extra 
information somewhere; but as there are only two bits of data (bus and 
addr), and agree that I need both, I'm having a hard time telling what 
you think is redundant / could be removed..
"hostdev" is an element of the union; so the structure should unpack 
like this:
struct {
  libxl_domid backend_domid;
  char * backend_domname;
  libxl_usb_device_type type;
  union u {
   struct {
    int hostbus, hostaddr;
   } hostdev;
  };
};

At the moment, "type" can only be "hostdev"; but I'm envisioning in the 
future that "type" might be "tablet", "mouse", "keyboard", maybe 
"mass-storage", and that the union would have more elements.
Does that clear things up?  Or am I totally confused? :-)

 -George

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