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

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

[George Dunlap]

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@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>

Changes in v1 (since the RFC):

- Use <class> rather than <type>, and <level> rather than specifying
  controller and device.  The idea being to allow SCSI to use
  terminology more natural to it (i.e., scsihost, scsitarget, scsilun)
  rather than naming things after USB (controller & device).

- Do not require each <level> to have a deviceid, but just a unique
  naming schema.

- Allow multiple levels.

- Include the paragraph about domain configuration lists.
---
 tools/libxl/libxl.h | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index 6b73848..46bcfe8 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -1396,6 +1396,71 @@ 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 classes of device,
+ * however, like USB or SCSI, inherently have the need to have a
+ * heiarchy of different levels, with lower-level devices "attached"
+ * to higher-level ones.  USB for instance has "controllers" at the
+ * top, which have busses, on which are devices, which consist of
+ * multiple interfaces.  SCSI has "hosts" at the top, then busses,
+ * targets, and LUNs.
+ *
+ * In that case, for each <class>, there will be a set of funcitons
+ * and types for each <level>.  For example, for <class>=usb, there
+ * may be <levels> ctrl (controller) and dev (device), with ctrl being
+ * level 0.
+ *
+ * libxl_device_<class><level0>_<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><level0> libxl_devid namespace.
+ *
+ * Lower-level devices must have a unique way to be identified.  One
+ * way to do this would be to name it via the name of the next level
+ * up plus an index; for instance, <ctrl devid, port number>.  Another
+ * way would be to have another devid namespace for that level.  This
+ * identifier will be used for queries and removals.
+ *
+ * Lower-level devices will will include in their
+ * libxl_device_<class><level> struct a field referring to the unique
+ * index of the level above.  For instance, libxl_device_usbdev might
+ * contain the controller devid.
+ *
+ * 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_<class><level>_add receives an empty reference to
+ * the level above, it may return an error.  Or it may (but is not
+ * required to) automatically choose a suitable device in the level
+ * above to which to attach the new device at this level.  It may also
+ * (but is not required to) automatically create a new device at the
+ * level above if no suitable devices exist.  Each class should
+ * document its behavior.
+ *
+ * libxl_device_<class><level>_list will list all devices of <class>
+ * at <level> in the domain.  For example, libxl_class_usbctrl_list
+ * will list all usb controllers; libxl_class_usbdev_list will list
+ * all usb devices across all controllers.
+ *
+ * For each class, the domain config file will contain a single list
+ * for each level.  libxl will first iterate through the list of
+ * top-level devices, then iterate through each level down in turn,
+ * adding devices to devices in the level above.  For instance, there
+ * will be one list for all usb controllers, and one list for all usb
+ * devices.
+ * 
+ * If libxl_device_<class><level>_add automatically creates
+ * higher-level devices as necessary, then it is permissible for the
+ * higher-level lists to be empty and the device list to have devices
+ * with the field containing a reference to the higher level device
+ * uninitialized.
  */
 
 /* Disks */
-- 
2.1.4

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

[Juergen Gross]

On 24/11/15 15:40, 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@citrix.com>

Some typos (see below), otherwise:

Acked-by: Juergen Gross <jgross@suse.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>
> 
> Changes in v1 (since the RFC):
> 
> - Use <class> rather than <type>, and <level> rather than specifying
>   controller and device.  The idea being to allow SCSI to use
>   terminology more natural to it (i.e., scsihost, scsitarget, scsilun)
>   rather than naming things after USB (controller & device).
> 
> - Do not require each <level> to have a deviceid, but just a unique
>   naming schema.
> 
> - Allow multiple levels.
> 
> - Include the paragraph about domain configuration lists.
> ---
>  tools/libxl/libxl.h | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
> 
> diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
> index 6b73848..46bcfe8 100644
> --- a/tools/libxl/libxl.h
> +++ b/tools/libxl/libxl.h
> @@ -1396,6 +1396,71 @@ 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 classes of device,
> + * however, like USB or SCSI, inherently have the need to have a
> + * heiarchy of different levels, with lower-level devices "attached"

hierarchy

> + * to higher-level ones.  USB for instance has "controllers" at the
> + * top, which have busses, on which are devices, which consist of

buses?

> + * multiple interfaces.  SCSI has "hosts" at the top, then busses,
> + * targets, and LUNs.
> + *
> + * In that case, for each <class>, there will be a set of funcitons

functions

> + * and types for each <level>.  For example, for <class>=usb, there
> + * may be <levels> ctrl (controller) and dev (device), with ctrl being
> + * level 0.
> + *
> + * libxl_device_<class><level0>_<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><level0> libxl_devid namespace.
> + *
> + * Lower-level devices must have a unique way to be identified.  One
> + * way to do this would be to name it via the name of the next level
> + * up plus an index; for instance, <ctrl devid, port number>.  Another
> + * way would be to have another devid namespace for that level.  This
> + * identifier will be used for queries and removals.
> + *
> + * Lower-level devices will will include in their
> + * libxl_device_<class><level> struct a field referring to the unique
> + * index of the level above.  For instance, libxl_device_usbdev might
> + * contain the controller devid.
> + *
> + * 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_<class><level>_add receives an empty reference to
> + * the level above, it may return an error.  Or it may (but is not
> + * required to) automatically choose a suitable device in the level
> + * above to which to attach the new device at this level.  It may also
> + * (but is not required to) automatically create a new device at the
> + * level above if no suitable devices exist.  Each class should
> + * document its behavior.
> + *
> + * libxl_device_<class><level>_list will list all devices of <class>
> + * at <level> in the domain.  For example, libxl_class_usbctrl_list
> + * will list all usb controllers; libxl_class_usbdev_list will list
> + * all usb devices across all controllers.
> + *
> + * For each class, the domain config file will contain a single list
> + * for each level.  libxl will first iterate through the list of
> + * top-level devices, then iterate through each level down in turn,
> + * adding devices to devices in the level above.  For instance, there
> + * will be one list for all usb controllers, and one list for all usb
> + * devices.
> + * 
> + * If libxl_device_<class><level>_add automatically creates
> + * higher-level devices as necessary, then it is permissible for the
> + * higher-level lists to be empty and the device list to have devices
> + * with the field containing a reference to the higher level device
> + * uninitialized.
>   */
>  
>  /* Disks */
> 

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

[Chun Yan Liu]

>>> On 11/24/2015 at 10:40 PM, in message
<1448376011-20217-1-git-send-email-george.dunlap@eu.citrix.com>, George Dunlap
<george.dunlap@eu.citrix.com> 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.

Some typos. Otherwise, agreed.

- Chunyan
 
>  
> Signed-off-by: George Dunlap <george.dunlap@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> 
>  
> Changes in v1 (since the RFC): 
>  
> - Use <class> rather than <type>, and <level> rather than specifying 
>   controller and device.  The idea being to allow SCSI to use 
>   terminology more natural to it (i.e., scsihost, scsitarget, scsilun) 
>   rather than naming things after USB (controller & device). 
>  
> - Do not require each <level> to have a deviceid, but just a unique 
>   naming schema. 
>  
> - Allow multiple levels. 
>  
> - Include the paragraph about domain configuration lists. 
> --- 
>  tools/libxl/libxl.h | 65  
> +++++++++++++++++++++++++++++++++++++++++++++++++++++ 
>  1 file changed, 65 insertions(+) 
>  
> diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h 
> index 6b73848..46bcfe8 100644 
> --- a/tools/libxl/libxl.h 
> +++ b/tools/libxl/libxl.h 
> @@ -1396,6 +1396,71 @@ 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 classes of device, 
> + * however, like USB or SCSI, inherently have the need to have a 
> + * heiarchy of different levels, with lower-level devices "attached" 
> + * to higher-level ones.  USB for instance has "controllers" at the 
> + * top, which have busses, on which are devices, which consist of 
> + * multiple interfaces.  SCSI has "hosts" at the top, then busses, 
> + * targets, and LUNs. 
> + * 
> + * In that case, for each <class>, there will be a set of funcitons
                                                                                    ^^^ functions
> + * and types for each <level>.  For example, for <class>=usb, there 
> + * may be <levels> ctrl (controller) and dev (device), with ctrl being 
> + * level 0. 
> + * 
> + * libxl_device_<class><level0>_<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><level0> libxl_devid namespace. 
         <class><level0> ?
> + * 
> + * Lower-level devices must have a unique way to be identified.  One 
> + * way to do this would be to name it via the name of the next level 
> + * up plus an index; for instance, <ctrl devid, port number>.  Another 
> + * way would be to have another devid namespace for that level.  This 
> + * identifier will be used for queries and removals. 
> + * 
> + * Lower-level devices will will include in their 
                                      ^^^^^ s/will will/will/
> + * libxl_device_<class><level> struct a field referring to the unique 
> + * index of the level above.  For instance, libxl_device_usbdev might 
> + * contain the controller devid. 
> + * 
> + * 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_<class><level>_add receives an empty reference to 
> + * the level above, it may return an error.  Or it may (but is not 
> + * required to) automatically choose a suitable device in the level 
> + * above to which to attach the new device at this level.  It may also 
> + * (but is not required to) automatically create a new device at the 
> + * level above if no suitable devices exist.  Each class should 
> + * document its behavior. 
> + * 
> + * libxl_device_<class><level>_list will list all devices of <class> 
> + * at <level> in the domain.  For example, libxl_class_usbctrl_list 
                                                                  libxl_device_usbctrl_list
> + * will list all usb controllers; libxl_class_usbdev_list will list 
                                               libxl_device_usbdev_list
> + * all usb devices across all controllers. 
> + * 
> + * For each class, the domain config file will contain a single list 
> + * for each level.  libxl will first iterate through the list of 
> + * top-level devices, then iterate through each level down in turn, 
> + * adding devices to devices in the level above.  For instance, there 
> + * will be one list for all usb controllers, and one list for all usb 
> + * devices. 
> + *  
> + * If libxl_device_<class><level>_add automatically creates 
> + * higher-level devices as necessary, then it is permissible for the 
> + * higher-level lists to be empty and the device list to have devices 
> + * with the field containing a reference to the higher level device 
> + * uninitialized. 
>   */ 
>   
>  /* Disks */ 

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

[George Dunlap]

On Wed, Nov 25, 2015 at 3:11 AM, Chun Yan Liu <cyliu@suse.com> wrote:
>
>
>>>> On 11/24/2015 at 10:40 PM, in message
> <1448376011-20217-1-git-send-email-george.dunlap@eu.citrix.com>, George Dunlap
> <george.dunlap@eu.citrix.com> 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.
>
> Some typos. Otherwise, agreed.

Thanks.  If I fix the typos you've pointed out, can I add your Acked-by? :-)

 -George


>
> - Chunyan
>
>>
>> Signed-off-by: George Dunlap <george.dunlap@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>
>>
>> Changes in v1 (since the RFC):
>>
>> - Use <class> rather than <type>, and <level> rather than specifying
>>   controller and device.  The idea being to allow SCSI to use
>>   terminology more natural to it (i.e., scsihost, scsitarget, scsilun)
>>   rather than naming things after USB (controller & device).
>>
>> - Do not require each <level> to have a deviceid, but just a unique
>>   naming schema.
>>
>> - Allow multiple levels.
>>
>> - Include the paragraph about domain configuration lists.
>> ---
>>  tools/libxl/libxl.h | 65
>> +++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 65 insertions(+)
>>
>> diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
>> index 6b73848..46bcfe8 100644
>> --- a/tools/libxl/libxl.h
>> +++ b/tools/libxl/libxl.h
>> @@ -1396,6 +1396,71 @@ 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 classes of device,
>> + * however, like USB or SCSI, inherently have the need to have a
>> + * heiarchy of different levels, with lower-level devices "attached"
>> + * to higher-level ones.  USB for instance has "controllers" at the
>> + * top, which have busses, on which are devices, which consist of
>> + * multiple interfaces.  SCSI has "hosts" at the top, then busses,
>> + * targets, and LUNs.
>> + *
>> + * In that case, for each <class>, there will be a set of funcitons
>                                                                                     ^^^ functions
>> + * and types for each <level>.  For example, for <class>=usb, there
>> + * may be <levels> ctrl (controller) and dev (device), with ctrl being
>> + * level 0.
>> + *
>> + * libxl_device_<class><level0>_<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><level0> libxl_devid namespace.
>          <class><level0> ?
>> + *
>> + * Lower-level devices must have a unique way to be identified.  One
>> + * way to do this would be to name it via the name of the next level
>> + * up plus an index; for instance, <ctrl devid, port number>.  Another
>> + * way would be to have another devid namespace for that level.  This
>> + * identifier will be used for queries and removals.
>> + *
>> + * Lower-level devices will will include in their
>                                       ^^^^^ s/will will/will/
>> + * libxl_device_<class><level> struct a field referring to the unique
>> + * index of the level above.  For instance, libxl_device_usbdev might
>> + * contain the controller devid.
>> + *
>> + * 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_<class><level>_add receives an empty reference to
>> + * the level above, it may return an error.  Or it may (but is not
>> + * required to) automatically choose a suitable device in the level
>> + * above to which to attach the new device at this level.  It may also
>> + * (but is not required to) automatically create a new device at the
>> + * level above if no suitable devices exist.  Each class should
>> + * document its behavior.
>> + *
>> + * libxl_device_<class><level>_list will list all devices of <class>
>> + * at <level> in the domain.  For example, libxl_class_usbctrl_list
>                                                                   libxl_device_usbctrl_list
>> + * will list all usb controllers; libxl_class_usbdev_list will list
>                                                libxl_device_usbdev_list
>> + * all usb devices across all controllers.
>> + *
>> + * For each class, the domain config file will contain a single list
>> + * for each level.  libxl will first iterate through the list of
>> + * top-level devices, then iterate through each level down in turn,
>> + * adding devices to devices in the level above.  For instance, there
>> + * will be one list for all usb controllers, and one list for all usb
>> + * devices.
>> + *
>> + * If libxl_device_<class><level>_add automatically creates
>> + * higher-level devices as necessary, then it is permissible for the
>> + * higher-level lists to be empty and the device list to have devices
>> + * with the field containing a reference to the higher level device
>> + * uninitialized.
>>   */
>>
>>  /* Disks */
>
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

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

[Ian Campbell]

On Tue, 2015-11-24 at 14:40 +0000, 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@citrix.com>

With the previously reported (by Juergen and Chyn Yan) typoes fixed:

Acked-by: Ian Campbell <ian.campbell@citrix.com>> 

FWIW (which may not be much) some dev types (e.g. PCI) collapse one or more
of your "<levels>" into a single one by having multiple devid things
encoded in one way or another in the struct, e.g. libxl_device_pci has bus,
device, func etc as fields.

Not sure if that is worth mentioning (or encouraging) as a possibility for
other device types (assuming it makes sense in that schema)

> ---
> 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>
> 
> Changes in v1 (since the RFC):
> 
> - Use <class> rather than <type>, and <level> rather than specifying
>   controller and device.  The idea being to allow SCSI to use
>   terminology more natural to it (i.e., scsihost, scsitarget, scsilun)
>   rather than naming things after USB (controller & device).
> 
> - Do not require each <level> to have a deviceid, but just a unique
>   naming schema.
> 
> - Allow multiple levels.
> 
> - Include the paragraph about domain configuration lists.
> ---
>  tools/libxl/libxl.h | 65
> +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
> 
> diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
> index 6b73848..46bcfe8 100644
> --- a/tools/libxl/libxl.h
> +++ b/tools/libxl/libxl.h
> @@ -1396,6 +1396,71 @@ 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 classes of device,
> + * however, like USB or SCSI, inherently have the need to have a
> + * heiarchy of different levels, with lower-level devices "attached"
> + * to higher-level ones.  USB for instance has "controllers" at the
> + * top, which have busses, on which are devices, which consist of
> + * multiple interfaces.  SCSI has "hosts" at the top, then busses,
> + * targets, and LUNs.
> + *
> + * In that case, for each <class>, there will be a set of funcitons
> + * and types for each <level>.  For example, for <class>=usb, there
> + * may be <levels> ctrl (controller) and dev (device), with ctrl being
> + * level 0.
> + *
> + * libxl_device_<class><level0>_<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><level0> libxl_devid namespace.
> + *
> + * Lower-level devices must have a unique way to be identified.  One
> + * way to do this would be to name it via the name of the next level
> + * up plus an index; for instance, <ctrl devid, port number>.  Another
> + * way would be to have another devid namespace for that level.  This
> + * identifier will be used for queries and removals.
> + *
> + * Lower-level devices will will include in their
> + * libxl_device_<class><level> struct a field referring to the unique
> + * index of the level above.  For instance, libxl_device_usbdev might
> + * contain the controller devid.
> + *
> + * 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_<class><level>_add receives an empty reference to
> + * the level above, it may return an error.  Or it may (but is not
> + * required to) automatically choose a suitable device in the level
> + * above to which to attach the new device at this level.  It may also
> + * (but is not required to) automatically create a new device at the
> + * level above if no suitable devices exist.  Each class should
> + * document its behavior.
> + *
> + * libxl_device_<class><level>_list will list all devices of <class>
> + * at <level> in the domain.  For example, libxl_class_usbctrl_list
> + * will list all usb controllers; libxl_class_usbdev_list will list
> + * all usb devices across all controllers.
> + *
> + * For each class, the domain config file will contain a single list
> + * for each level.  libxl will first iterate through the list of
> + * top-level devices, then iterate through each level down in turn,
> + * adding devices to devices in the level above.  For instance, there
> + * will be one list for all usb controllers, and one list for all usb
> + * devices.
> + * 
> + * If libxl_device_<class><level>_add automatically creates
> + * higher-level devices as necessary, then it is permissible for the
> + * higher-level lists to be empty and the device list to have devices
> + * with the field containing a reference to the higher level device
> + * uninitialized.
>   */
>  
>  /* Disks */

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