xc_save: ignore the first suspend event channel notification

I've noticed that the suspend event channel becomes pending as soon as
it is bound. I'm not sure why or whether this is intentional, but it
means that the suspend function will return before the domain has
completed suspending unless the first notification is cleared. Without
this patch, xc_domain_save may find that the guest has not suspended
and sleep in 10ms chunks until it does. Typically this is several
milliseconds of wasted time.

From: Brendan Cully <brendan@cs.ubc.ca>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

1 files changed, 47 insertions, 31 deletions

diff --git a/tools/xcutils/xc_save.c b/tools/xcutils/xc_save.c
index 3dfc03117a..c8b0ccb05c 100644
--- a/tools/xcutils/xc_save.c
+++ b/tools/xcutils/xc_save.c
@@ -46,17 +46,36 @@ static int compat_suspend(void)
 static int suspend_evtchn_release(void)
 {
     if (si.suspend_evtchn >= 0) {
-	xc_evtchn_unbind(si.xce, si.suspend_evtchn);
-	si.suspend_evtchn = -1;
+        xc_evtchn_unbind(si.xce, si.suspend_evtchn);
+        si.suspend_evtchn = -1;
     }
     if (si.xce >= 0) {
-	xc_evtchn_close(si.xce);
-	si.xce = -1;
+        xc_evtchn_close(si.xce);
+        si.xce = -1;
     }
 
     return 0;
 }
 
+static int await_suspend(void)
+{
+    int rc;
+
+    do {
+        rc = xc_evtchn_pending(si.xce);
+        if (rc < 0) {
+            warnx("error polling suspend notification channel: %d", rc);
+            return -1;
+        }
+    } while (rc != si.suspend_evtchn);
+
+    /* harmless for one-off suspend */
+    if (xc_evtchn_unmask(si.xce, si.suspend_evtchn) < 0)
+        warnx("failed to unmask suspend notification channel: %d", rc);
+
+    return 0;
+}
+
 static int suspend_evtchn_init(int xc, int domid)
 {
     struct xs_handle *xs;
@@ -71,16 +90,16 @@ static int suspend_evtchn_init(int xc, int domid)
 
     xs = xs_daemon_open();
     if (!xs) {
-	warnx("failed to get xenstore handle");
-	return -1;
+        warnx("failed to get xenstore handle");
+        return -1;
     }
     sprintf(path, "/local/domain/%d/device/suspend/event-channel", domid);
     portstr = xs_read(xs, XBT_NULL, path, &plen);
     xs_daemon_close(xs);
 
     if (!portstr || !plen) {
-	warnx("could not read suspend event channel");
-	return -1;
+        warnx("could not read suspend event channel");
+        return -1;
     }
 
     port = atoi(portstr);
@@ -88,22 +107,25 @@ static int suspend_evtchn_init(int xc, int domid)
 
     si.xce = xc_evtchn_open();
     if (si.xce < 0) {
-	warnx("failed to open event channel handle");
-	goto cleanup;
+        warnx("failed to open event channel handle");
+        goto cleanup;
     }
 
     si.suspend_evtchn = xc_evtchn_bind_interdomain(si.xce, domid, port);
     if (si.suspend_evtchn < 0) {
-	warnx("failed to bind suspend event channel: %d", si.suspend_evtchn);
-	goto cleanup;
+        warnx("failed to bind suspend event channel: %d", si.suspend_evtchn);
+        goto cleanup;
     }
 
     rc = xc_domain_subscribe_for_suspend(xc, domid, port);
     if (rc < 0) {
-	warnx("failed to subscribe to domain: %d", rc);
-	goto cleanup;
+        warnx("failed to subscribe to domain: %d", rc);
+        goto cleanup;
     }
 
+    /* event channel is pending immediately after binding */
+    await_suspend();
+
     return 0;
 
   cleanup:
@@ -121,21 +143,14 @@ static int evtchn_suspend(void)
 
     rc = xc_evtchn_notify(si.xce, si.suspend_evtchn);
     if (rc < 0) {
-	warnx("failed to notify suspend request channel: %d", rc);
-	return 0;
+        warnx("failed to notify suspend request channel: %d", rc);
+        return 0;
     }
 
-    do {
-      rc = xc_evtchn_pending(si.xce);
-      if (rc < 0) {
-	warnx("error polling suspend notification channel: %d", rc);
-	return 0;
-      }
-    } while (rc != si.suspend_evtchn);
-
-    /* harmless for one-off suspend */
-    if (xc_evtchn_unmask(si.xce, si.suspend_evtchn) < 0)
-	warnx("failed to unmask suspend notification channel: %d", rc);
+    if (await_suspend() < 0) {
+        warnx("suspend failed");
+        return 0;
+    }
 
     /* notify xend that it can do device migration */
     printf("suspended\n");
@@ -147,7 +162,7 @@ static int evtchn_suspend(void)
 static int suspend(void)
 {
     if (si.suspend_evtchn >= 0)
-	return evtchn_suspend();
+        return evtchn_suspend();
 
     return compat_suspend();
 }
@@ -194,7 +209,7 @@ static void qemu_flip_buffer(int domid, int next_active)
      * other buffer */
     if (!xs_write(xs, XBT_NULL, qemu_next_active_path, &digit, 1))
         errx(1, "can't write next-active to store path (%s)\n", 
-	     qemu_next_active_path);
+             qemu_next_active_path);
 
     /* Wait a while for qemu to signal that it has switched to the new 
      * active buffer */
@@ -286,7 +301,7 @@ main(int argc, char **argv)
     int ret;
 
     if (argc != 6)
-	errx(1, "usage: %s iofd domid maxit maxf flags", argv[0]);
+        errx(1, "usage: %s iofd domid maxit maxf flags", argv[0]);
 
     xc_fd = xc_interface_open();
     if (xc_fd < 0)
@@ -298,7 +313,8 @@ main(int argc, char **argv)
     max_f = atoi(argv[4]);
     flags = atoi(argv[5]);
 
-    suspend_evtchn_init(xc_fd, domid);
+    if (suspend_evtchn_init(xc_fd, domid) < 0)
+        warnx("suspend event channel initialization failed, using slow path");
 
     ret = xc_domain_save(xc_fd, io_fd, domid, maxit, max_f, flags, 
                          &suspend, !!(flags & XCFLAGS_HVM),