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

[George Dunlap <george.dunlap@eu.citrix.com>]

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