[Xen-devel] [PATCH 2/3] libxl: make creation of xenstore suspend event channel node optional

[Paul Durrant <pdurrant@xxxxxxxxxx>]

The purpose and semantics of the node are explained in
xenstore-paths.pandoc [1]. It was originally introduced in xend by commit
17636f47a474 "Teach xc_save to use event-channel-based domain suspend if
available.". Note that, because, the top-level frontend 'device' node was
created writable by the guest in xend, there was no need to explicitly
create the 'suspend-event-channel' node as writable node.

However, libxl creates the 'device' node as read-only by the guest and so
explicit creation of the 'suspend-event-channel' node is necessary to make
it usable. This unfortunately has the side-effect of making some old
Windows PV drivers [2] cease to function. This is because they scan the top
level 'device' node, find the 'suspend' node and expect it to contain the
usual sub-nodes describing a PV frontend. When this is found not to be the
case, enumeration ceases and (because the 'suspend' node is observed before
the 'vbd' node) no system disk is enumerated. Windows will then crash with
bugcheck code 0x7B.

This patch adds a boolean 'suspend_event_channel' field into
libxl_create_info to control whether the xenstore node is created and a
similarly named option in xl.cfg which, for compatibility with previous
libxl behaviour, defaults to true.

[1] 
https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/xenstore-paths.pandoc;hb=HEAD#l177
[2] 
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/5/html/para-virtualized_windows_drivers_guide/sect-para-virtualized_windows_drivers_guide-installing_and_configuring_the_para_virtualized_drivers-installing_the_para_virtualized_drivers

NOTE: While adding the new LIBXL_HAVE_CREATEINFO_SUSPEND_EVENT_CHANNEL
      definition into libxl.h, this patch corrects the previous stanza
      which erroneously implies ibxl_domain_create_info is a function.

Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
---
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>
Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>
---
 docs/man/xl.cfg.5.pod.in    |  7 +++++++
 tools/libxl/libxl.h         | 13 ++++++++++++-
 tools/libxl/libxl_create.c  | 12 +++++++++---
 tools/libxl/libxl_types.idl |  1 +
 tools/xl/xl_parse.c         |  3 +++
 5 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
index 0cad561375..5f476f1e1d 100644
--- a/docs/man/xl.cfg.5.pod.in
+++ b/docs/man/xl.cfg.5.pod.in
@@ -668,6 +668,13 @@ file.
 
 =back
 
+=item B<suspend_event_channel=BOOLEAN>
+
+Create the xenstore path for the domain's suspend event channel. The
+existence of this path can cause problems with older PV drivers running
+in the guest. If this option is not specified then it will default to
+B<true>.
+
 =back
 
 =head2 Devices
diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index 35e13428b2..d2afe48512 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -1272,10 +1272,21 @@ void libxl_mac_copy(libxl_ctx *ctx, libxl_mac *dst, 
const libxl_mac *src);
  * LIBXL_HAVE_CREATEINFO_DOMID
  *
  * libxl_domain_create_new() and libxl_domain_create_restore() will use
- * a domid specified in libxl_domain_create_info().
+ * a domid specified in libxl_domain_create_info.
  */
 #define LIBXL_HAVE_CREATEINFO_DOMID
 
+/*
+ * LIBXL_HAVE_CREATEINFO_SUSPEND_EVENT_CHANNEL
+ *
+ * libxl_domain_create_info contains a boolean 'suspend_event_channel'
+ * value to control whether the xenstore path:
+ *
+ * /local/domain/$DOMID/device/suspend/event-channel (RW)
+ *
+ * is created.
+ */
+
 typedef char **libxl_string_list;
 void libxl_string_list_dispose(libxl_string_list *sl);
 int libxl_string_list_length(const libxl_string_list *sl);
diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index 27627cb199..7119e95412 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -57,6 +57,8 @@ int libxl__domain_create_info_setdefault(libxl__gc *gc,
     if (!c_info->ssidref)
         c_info->ssidref = SECINITSID_DOMU;
 
+    libxl_defbool_setdefault(&c_info->suspend_event_channel, true);
+
     return 0;
 }
 
@@ -782,9 +784,13 @@ retry_transaction:
     libxl__xs_mknod(gc, t,
                     GCSPRINTF("%s/control/sysrq", dom_path),
                     rwperm, ARRAY_SIZE(rwperm));
-    libxl__xs_mknod(gc, t,
-                    GCSPRINTF("%s/device/suspend/event-channel", dom_path),
-                    rwperm, ARRAY_SIZE(rwperm));
+
+    if (libxl_defbool_val(info->suspend_event_channel))
+        libxl__xs_mknod(gc, t,
+                        GCSPRINTF("%s/device/suspend/event-channel",
+                                  dom_path),
+                        rwperm, ARRAY_SIZE(rwperm));
+
     libxl__xs_mknod(gc, t,
                     GCSPRINTF("%s/data", dom_path),
                     rwperm, ARRAY_SIZE(rwperm));
diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
index d0d431614f..2bce19bcf0 100644
--- a/tools/libxl/libxl_types.idl
+++ b/tools/libxl/libxl_types.idl
@@ -418,6 +418,7 @@ libxl_domain_create_info = Struct("domain_create_info",[
     ("run_hotplug_scripts",libxl_defbool),
     ("driver_domain",libxl_defbool),
     ("passthrough",  libxl_passthrough),
+    ("suspend_event_channel",libxl_defbool),
     ], dir=DIR_IN)
 
 libxl_domain_restore_params = Struct("domain_restore_params", [
diff --git a/tools/xl/xl_parse.c b/tools/xl/xl_parse.c
index b881184804..122c6eb641 100644
--- a/tools/xl/xl_parse.c
+++ b/tools/xl/xl_parse.c
@@ -2725,6 +2725,9 @@ skip_usbdev:
 
     parse_vkb_list(config, d_config);
 
+    xlu_cfg_get_defbool(config, "suspend_event_channel",
+                        &c_info->suspend_event_channel, 0);
+
     xlu_cfg_destroy(config);
 }
 
-- 
2.20.1


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