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

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

On 05/21/2015 06:07 PM, 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.
> 
> Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
> ---
> CC: Ian Campbell <ian.campbell@citrix.com>
> CC: Ian Jackson <ian.jackson@citrix.com>
> CC: Wei Liu <wei.liu2@citrix.com>
> CC: Juergen Gross <jgross@suse.com>
> CC: Chun Yan Liu <cyliu@suse.com>
> CC: Olaf Hering <olaf@aepfle.de>
> 
> 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.

Hmm, I also meant to add:

---
For each type, the domain config file will contain a single list of
controllers, and a single list of devices.  libxl will first iterate
through the list adding the controlllers, then iterate through the list
adding each device to the ctrl listed.  If libxl_device_<type>_add
automatically creates controllers as necessary, then it is permissible
for the controller list to be empty and the device list to have devices
(with the ctrl field uninitialized).
---

 -George

>   */
>  
>  /* Disks */
>