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Re: [Qemu-devel] [PATCH v8 3/3] qmp hmp: Make system_wakeup check wake-u

From: Daniel Henrique Barboza
Subject: Re: [Qemu-devel] [PATCH v8 3/3] qmp hmp: Make system_wakeup check wake-up support and run state
Date: Thu, 6 Sep 2018 07:54:36 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 09/05/2018 09:27 PM, Michael Roth wrote:
Quoting Daniel Henrique Barboza (2018-07-05 15:08:13)
The qmp/hmp command 'system_wakeup' is simply a direct call to
'qemu_system_wakeup_request' from vl.c. This function verifies if
runstate is SUSPENDED and if the wake up reason is valid before
proceeding. However, no error or warning is thrown if any of those
pre-requirements isn't met. There is no way for the caller to
differentiate between a successful wakeup or an error state caused
when trying to wake up a guest that wasn't suspended.

This means that system_wakeup is silently failing, which can be
considered a bug. Adding error handling isn't an API break in this
case - applications that didn't check the result will remain broken,
the ones that check it will have a chance to deal with it.

Adding to that, the commit before previous created a new QMP API called
query-current-machine, with a new flag called wakeup-suspend-support,
that indicates if the guest has the capability of waking up from suspended
state. Although such guest will never reach SUSPENDED state and erroring
it out in this scenario would suffice, it is more informative for the user
to differentiate between a failure because the guest isn't suspended versus
a failure because the guest does not have support for wake up at all.

All this considered, this patch changes qmp_system_wakeup to:

- check if the guest has the capability to wake-up from suspend. If
not, error out informing about the lack of support;

- make the runstate verification before proceeding to call
qemu_system_wakeup_request, firing up an error message if the user tries
to wake up a machine that isn't in SUSPENDED state.

After this patch, this is the output of system_wakeup in a guest that
does not have wake-up from suspend support (ppc64):

(qemu) system_wakeup
wake-up from suspend is not supported by this guest

And this is the output of system_wakeup in a x86 guest that has the
support but isn't suspended:

(qemu) system_wakeup
Unable to wake up: guest is not in suspended state

Reported-by: Balamuruhan S <address@hidden>
Signed-off-by: Daniel Henrique Barboza <address@hidden>
  hmp.c                   |  5 ++++-
  include/sysemu/sysemu.h |  1 +
  qapi/misc.json          |  5 ++++-
  qmp.c                   | 11 ++++++++++-
  vl.c                    |  2 +-
  5 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/hmp.c b/hmp.c
index 41f5e39b72..83a8041d11 100644
--- a/hmp.c
+++ b/hmp.c
@@ -1169,7 +1169,10 @@ void hmp_cont(Monitor *mon, const QDict *qdict)

  void hmp_system_wakeup(Monitor *mon, const QDict *qdict)
-    qmp_system_wakeup(NULL);
+    Error *err = NULL;
+    qmp_system_wakeup(&err);
+    hmp_handle_error(mon, &err);

  void hmp_nmi(Monitor *mon, const QDict *qdict)
diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
index b921c6f3b7..483f804b36 100644
--- a/include/sysemu/sysemu.h
+++ b/include/sysemu/sysemu.h
@@ -70,6 +70,7 @@ void qemu_exit_preconfig_request(void);
  void qemu_system_reset_request(ShutdownCause reason);
  void qemu_system_suspend_request(void);
  void qemu_register_suspend_notifier(Notifier *notifier);
+bool qemu_wakeup_suspend_support(void);
  void qemu_system_wakeup_request(WakeupReason reason);
  void qemu_system_wakeup_enable(WakeupReason reason, bool enabled);
  void qemu_register_wakeup_notifier(Notifier *notifier);
diff --git a/qapi/misc.json b/qapi/misc.json
index 266f88cb09..86195d79e9 100644
--- a/qapi/misc.json
+++ b/qapi/misc.json
@@ -1230,7 +1230,10 @@
  # @system_wakeup:
-# Wakeup guest from suspend.  Does nothing in case the guest isn't suspended.
+# If the guest has wake-up from suspend support enabled
+# (wakeup-suspend-support flag from query-current-machine), wakeup guest
+# from suspend if the guest is in SUSPENDED state. Returns an error
+# otherwise.
  # Since:  1.1
diff --git a/qmp.c b/qmp.c
index 73e46d795f..0673ff1fa5 100644
--- a/qmp.c
+++ b/qmp.c
@@ -215,7 +215,16 @@ void qmp_cont(Error **errp)

  void qmp_system_wakeup(Error **errp)
-    qemu_system_wakeup_request(QEMU_WAKEUP_REASON_OTHER);
+    if (!qemu_wakeup_suspend_support()) {
+        error_setg(errp,
+                   "wake-up from suspend is not supported by this guest");
+        return;
+    }
+    if (!runstate_check(RUN_STATE_SUSPENDED)) {
+        error_setg(errp,
+                   "Unable to wake up: guest is not in suspended state");
+        return;
+    }
Was removing qemu_system_wakeup_request() intentional or did that just get
lost somehow?

WOW! That was a mistake! Thanks for spotting it Mike! I'll re-insert it back
in the next spin.

  ObjectPropertyInfoList *qmp_qom_list(const char *path, Error **errp)
diff --git a/vl.c b/vl.c
index 96ad448d57..2af10b75cb 100644
--- a/vl.c
+++ b/vl.c
@@ -1769,7 +1769,7 @@ void qemu_register_wakeup_notifier(Notifier *notifier)
      notifier_list_add(&wakeup_notifiers, notifier);

-static bool qemu_wakeup_suspend_support(void)
+bool qemu_wakeup_suspend_support(void)
      return !QLIST_EMPTY(&wakeup_notifiers.notifiers) && acpi_enabled;

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