qemu-ga: add guest-suspend-disk

As the command name implies, this command suspends the guest to disk.

The suspend operation is implemented by two functions: bios_supports_mode()
and guest_suspend(). Both functions are generic enough to be used by
other suspend modes (introduced by next commits).

Both functions will try to use the scripts provided by the pm-utils
package if it's available. If it's not available, a manual method,
which consists of directly writing to '/sys/power/state', will be used.

To reap terminated children, a new signal handler is installed in the
parent to catch SIGCHLD signals and a non-blocking call to waitpid()
is done to collect their exit statuses. The statuses, however, are
discarded.

The approach used to query the guest for suspend support deserves some
explanation. It's implemented by bios_supports_mode() and shown below:

  qemu-ga
     |
 create pipe
     |
   fork()
     -----------------
     |               |
     |               |
     |             fork()
     |               --------------------------
     |               |                        |
     |               |                        |
     |               |               exec('pm-is-supported')
     |               |
     |              wait()
     |       write exit status to pipe
     |              exit
     |
  read pipe

This might look complex, but the resulting code is quite simple.
The purpose of that approach is to allow qemu-ga to reap its children
(semi-)automatically from its SIGCHLD handler.

Implementing this the obvious way, that's, doing the exec() call from
the first child process, would force us to introduce a more complex way
to reap qemu-ga's children. Like registering PIDs to be reaped and
having a way to wait for them when returning their exit status to
qemu-ga is necessary. The approach explained above avoids that complexity.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/qapi-schema-guest.json b/qapi-schema-guest.json
index 706925dea6..f4e0e1d241 100644
--- a/qapi-schema-guest.json
+++ b/qapi-schema-guest.json
@@ -295,3 +295,27 @@
 ##
 { 'command': 'guest-fsfreeze-thaw',
   'returns': 'int' }
+
+##
+# @guest-suspend-disk
+#
+# Suspend guest to disk.
+#
+# This command tries to execute the scripts provided by the pm-utils package.
+# If it's not available, the suspend operation will be performed by manually
+# writing to a sysfs file.
+#
+# For the best results it's strongly recommended to have the pm-utils
+# package installed in the guest.
+#
+# Returns: nothing on success
+#          If suspend to disk is not supported, Unsupported
+#
+# Notes: o This is an asynchronous request. There's no guarantee a response
+#          will be sent
+#        o It's strongly recommended to issue the guest-sync command before
+#          sending commands when the guest resumes
+#
+# Since: 1.1
+##
+{ 'command': 'guest-suspend-disk' }