Re: [Xen-devel] [PATCH RFC v1] libxl: Introduce a template for devices with a controller

[Juergen Gross <jgross@xxxxxxxx>]

On 06/17/2015 01:06 PM, Ian Campbell wrote:
> On Thu, 2015-05-21 at 18:07 +0100, George Dunlap wrote:
> > We have several outstanding patch series which add devices that have
> > two levels: a controller and individual devices attached to that
> > controller.
> >
> > In the interest of consistency, this patch introduces a section that
> > sketches out a template for interfaces for such devices.
> Chun Yan and Jeurgen:
>
> I was hoping we could come to some sort of agreement on this such that
> it can be used as the basis for both the pvusb and pvscsi interfaces. As
> such your feedback here is important...
I already gave some feedback. As everything regarding this feedback
has been discussed:

Acked-by: Juergen Gross <jgross@xxxxxxxx>

> > Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> > ---
> > CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
> > CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> > CC: Juergen Gross <jgross@xxxxxxxx>
> > CC: Chun Yan Liu <cyliu@xxxxxxxx>
> > CC: Olaf Hering <olaf@xxxxxxxxx>
> >
> > So, this is definitely RFC -- I tried to spec things out in a way that
> > made sense, but I often just chose something that I thought would be a
> > sensible starting point for discussion.
> >
> > This spec looks a lot more like the PVUSB spec than the PVSCSI spec,
> > in part because I think the PVUSB spec has already had a lot more
> > thought that's gone into it.
> >
> > A couple of random points to discuss:
> >
> > * Calling things "controllers", using <type>ctrl for the device name,
> >    and using "ctrl" as the field name for the devid of the controller
> >    in the individual devices.
> >
> > * I've said that having an index (port, lun, whatever) is optional.
> >    Do we want to make that requred?  Do we want it to have a consistent
> >    name?  In the case of emulated USB, we can't really specify to qemu
> >    what port the device gets attached to, so I'm tempted to say it's
> >    not required; but even there we could always give it a port number
> >    just for name's sake.
> >
> > * Naming sub-devices.  We need to have a way to uniquely name both
> >    controllers and subdevices.  Here I've said that we will have both
> >    <type>ctrl and <type> devid namespaces, mainly because in the
> >    previous point I opted not to require an index.  Another option
> >    would be not to have another devid namespace, but to use
> >    <ctrl,index> as the unique identifier.  (This would mean requiring
> >    the index/port/lun specification above.)
> >
> > * libxl_device_<type>_list listing devices across all controllers.  I
> >    think this is the most practical thing to do, but one could imagine
> >    wanting to query by controller ID instead.
> >
> > Feedback welcome.
> > ---
> >   tools/libxl/libxl.h | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 46 insertions(+)
> >
> > diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
> > index 2ed7194..d757845 100644
> > --- a/tools/libxl/libxl.h
> > +++ b/tools/libxl/libxl.h
> > @@ -1234,6 +1234,52 @@ void libxl_vtpminfo_list_free(libxl_vtpminfo *, int 
> > nr_vtpms);
> >    *
> >    *   This function does not interact with the guest and therefore
> >    *   cannot block on the guest.
> > + *
> > + * Controllers
> > + * -----------
> > + *
> > + * Most devices are treated individually.  Some devices however, like
> > + * USB or SCSI, inherently have the need to have "busses" or
> > + * "controllers" to which individual devices can be attached.
> > + *
> > + * In that case, for each <type>, there will be two sets of the
> > + * functions, types, and devid namespaces outlined above: one based on
> > + * '<type>', and one based on '<type>ctrl'.
> > + *
> > + * In those cases, libxl_device_<type>ctrl_<function> will act more or
> > + * less like top-level non-bus devices: they will either create or
> > + * accept a libxl_devid which will be unique within the
> > + * <type>ctrl libxl_devid namespace.
> > + *
> > + * Second-level devices which will be attached to a controller will
> > + * include in their libxl_device_<type> a field called ctrl, which
> > + * will be the libxl_devid of the corresponding controller.  It may also
> > + * include an index onto that bus, that represents (for example) a USB
> > + * port or a SCSI LUN.
> > + *
> > + * These second-level devices will also have their own devid which
> > + * will be unique within the <type> devid namespace, and will be used
> > + * for queries or removals.
> > + *
> > + * In the case where there are multiple different ways to implement a
> > + * given device -- for instance, one which is fully PV and one which
> > + * uses an emulator -- the controller will contain a field which
> > + * specifies what type of implementation is used.  The implementations
> > + * of individual devices will be known by the controller to which they are
> > + * attached.
> > + *
> > + * If libxl_device_<type>_add receives an uninitialized ctrl devid, it
> > + * may return an error.  Or it may (but is not required to) choose to
> > + * automatically choose a suitable controller to which to attach the
> > + * new device.  It may also (but is not required to) automatically
> > + * create a new controller if no suitable controllers exist.
> > + * Individual devices should document their behavior.
> > + *
> > + * libxl_device_<type>ctrl_list will list all controllers for the domain.
> > + *
> > + * libxl_device_<type>_list will list all devices for all controllers
> > + * for the domain.  The individual libxl_device_<type> will include
> > + * the devid of the controller to which it is attached.
> >    */
> >
> >   /* Disks */

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