Re: [Xen-devel] [PATCH v9 01/17] libxl: change ao_device_remove to ao_device

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Roger Pau Monne writes ("[PATCH v9 01/17] libxl: change ao_device_remove to 
ao_device"):
> Introduce a new structure to track state of device backends, that will
> be used in following patches on this series.
> 
> This structure if used for both device creation and device
> destruction and removes libxl__ao_device_remove.
> 
> Changes since v8:
> 
>  * Don't wait for QDISK, VKBD or VFB to disconnect, since Qemu doesn't
>    honour the disconnection protocol.

Following discussion in front of a whiteboard (thanks also to Ian C
and Stefano), we have concluded that this needs to be done
differently.  Here is the comment I promised Roger I would write

Ian.

Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>


/*
 * Algorithm for handling device removal (including domain
 * destruction).  This is somewhat subtle because we may already have
 * killed the domain and caused the death of qemu.
 *
 * In current versions of qemu there is no mechanism for ensuring that
 * the resources used by its devices (both emulated and any PV devices
 * provided by qemu) are freed (eg, fds closed) before it shuts down,
 * and no confirmation from a terminating qemu back to the toolstack.
 *
 * This will need to be fixed in Xen 4.3.  In the meantime (Xen 4.2)
 * we implement a bodge.
 *
 *      WE WANT TO UNPLUG         WE WANT TO SHUT DOWN OR DESTROY
 *                    |                           |
 *                    |             LIBXL SENDS SIGHUP TO QEMU
 *                    |      .....................|........................
 *                    |      : XEN 4.3+ PLANNED   |                       :
 *                    |      :      QEMU TEARS DOWN ALL DEVICES           :
 *                    |      :      FREES RESOURCES (closing fds)         :
 *                    |      :      SETS PV BACKENDS TO STATE 5,          :
 *                    |      :       waits for PV frontends to shut down  :
 *                    |      :       SETS PV BACKENDS TO STATE 6          :
 *                    |      :                    |                       :
 *                    |      :      QEMU NOTIFIES TOOLSTACK (via          :
 *                    |      :       xenstore) that it is exiting         :
 *                    |      :      QEMU EXITS (parent may be init)       :
 *                    |      :                    |                       :
 *                    |      :        TOOLSTACK WAITS FOR QEMU            :
 *                    |      :        notices qemu has finished           :
 *                    |      :....................|.......................:
 *                    |      .--------------------'
 *                    V      V
 *                  for each device
 *                 we want to unplug/remove
 *       ..................|...........................................
 *       :                 V                       XEN 4.2 RACY BODGE :
 *       :      device is provided by    qemu                         :
 *       :            |            `-----------.                      :
 *       :   something|                        V                      :
 *       :    else, eg|             domain (that is domain for which  :
 *       :     blkback|              this PV device is the backend,   :
 *       :            |              which might be the stub dm)      :
 *       :            |                is still alive?                :
 *       :            |                  |        |                   :
 *       :            |                  |alive   |dead               :
 *       :            |<-----------------'        |                   :
 *       :            |    hopefully qemu is      |                   :
 *       :            |       still running       |                   :
 *       :............|.................          |                   :
 *             ,----->|                :     we may be racing         :
 *             |    backend state?     :      with qemu's death       :
 *             ^      |         |      :          |                   :
 *     xenstore|      |other    |6     :      WAIT 2.0s               :
 *     conflict|      |         |      :       TIMEOUT                :
 *             |   WRITE B.E.   |      :          |                   :
 *             |    STATE:=5    |      :     hopefully qemu has       :
 *             `---'  |         |      :      gone by now and         :
 *                    |ok       |      :      freed its resources     :
 *                    |         |      :          |                   :
 *              WAIT FOR        |      :     SET B.E.                 :
 *              STATE==6        |      :      STATE:=6                :
 *              /     |         |      :..........|...................:
 *      timeout/    ok|         |                 |
 *            /       |         |                 |
 *           |    RUN HOTPLUG <-'<----------------'
 *           |      SCRIPT
 *           |        |
 *           `---> NUKE
 *                  BACKEND
 *                    |
 *                   DONE.
 */

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel