Re: [Xen-devel] [PATCH V4 3/7] libxl: add pvusb API

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2015-06-16 at 12:44 +0100, Ian Jackson wrote:
> George Dunlap writes ("Re: [Xen-devel] [PATCH V4 3/7] libxl: add pvusb API"):
> > On Tue, Jun 16, 2015 at 12:19 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
> > > I'm pretty sure this is just a matter of timing during boot: the busses
> > > are all (or at least some of them) queried at the same time and the
> > > first answering gets number 1, the next 2, and so on. And timing seems
> > > to be different enough to result in unstable bus numbers.
> > 
> > Right -- I meant "stable in topology within one boot", but at least
> > two of you have now understood me to mean "across reboots" by default,
> > so that's obviously a detail that needs to be specified. :-)
> 
> I think "stable in topology within one boot" is nearly vacuous (or are
> we expecting buses or devices to spontaneously renumber themselves for
> no reason) and also nearly useless.
> 
> We definitely need _some_ way for a user to identify a specific device
> uniquely in a way that is reliable from one boot to the next.  vid:pid
> is one way to do this, but not always sufficient.

We should perhaps separate out the "end user" from the "libxl
user" (i.e. the toolstack application) in our deliberations here.

We have the option of settling on a single way of describing a device to
libxl in the libxl API but allowing the toolstack to choose to present
the user with multiple different ways to specify things by providing a
suitable set of helper functions (in either libxlu or libxl proper) from
a variety of schemes to the one true way of describing a device.

That sounds better to me than having the libxl API consist of the union
of all potentially useful ways to refer to a device.

I'm not sure whether that also gives us the freedom to use something
which is only stable for a given boot at the libxl API layer though.

Ian.


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