xen: fix booting ballooned down hvm guest

Commit 96edd61dcf ("xen/balloon: don't
online new memory initially") introduced a regression when booting a
HVM domain with memory less than mem-max: instead of ballooning down
immediately the system would try to use the memory up to mem-max
resulting in Xen crashing the domain.

For HVM domains the current size will be reflected in Xenstore node
memory/static-max instead of memory/target.

Additionally we have to trigger the ballooning process at once.

Cc: <stable@vger.kernel.org> # 4.13
Fixes: 96edd61dcf ("xen/balloon: don't
       online new memory initially")

Reported-by: Simon Gaiser <hw42@ipsumj.de>
Suggested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xen-balloon.c

@@ -57,7 +57,7 @@ static int register_balloon(struct device *dev);
 static void watch_target(struct xenbus_watch *watch,
 			 const char *path, const char *token)
 {
-	unsigned long long new_target;
+	unsigned long long new_target, static_max;
 	int err;
 	static bool watch_fired;
 	static long target_diff;
@@ -72,13 +72,20 @@ static void watch_target(struct xenbus_watch *watch,
 	 * pages. PAGE_SHIFT converts bytes to pages, hence PAGE_SHIFT - 10.
 	 */
 	new_target >>= PAGE_SHIFT - 10;
-	if (watch_fired) {
-		balloon_set_new_target(new_target - target_diff);
-		return;
+
+	if (!watch_fired) {
+		watch_fired = true;
+		err = xenbus_scanf(XBT_NIL, "memory", "static-max", "%llu",
+				   &static_max);
+		if (err != 1)
+			static_max = new_target;
+		else
+			static_max >>= PAGE_SHIFT - 10;
+		target_diff = xen_pv_domain() ? 0
+				: static_max - balloon_stats.target_pages;
 	}
 
-	watch_fired = true;
-	target_diff = new_target - balloon_stats.target_pages;
+	balloon_set_new_target(new_target - target_diff);
 }
 static struct xenbus_watch target_watch = {
 	.node = "memory/target",