Do not drop global mutex for polled main loop runs

If we call select without a timeout, it's more efficient to keep the
global mutex locked as we may otherwise just play ping pong with a
vcpu thread contending for it. This is particularly important for TCG
mode where we run in lock-step with the vcpu thread.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

vl.c

diff --git a/vl.c b/vl.c
index 5f1a177..c52937a 100644
--- a/vl.c
+++ b/vl.c
@@ -1349,9 +1349,15 @@
     qemu_iohandler_fill(&nfds, &rfds, &wfds, &xfds);
     slirp_select_fill(&nfds, &rfds, &wfds, &xfds);
 
-    qemu_mutex_unlock_iothread();
+    if (timeout > 0) {
+        qemu_mutex_unlock_iothread();
+    }
+
     ret = select(nfds + 1, &rfds, &wfds, &xfds, &tv);
-    qemu_mutex_lock_iothread();
+
+    if (timeout > 0) {
+        qemu_mutex_lock_iothread();
+    }
 
     qemu_iohandler_poll(&rfds, &wfds, &xfds, ret);
     slirp_select_poll(&rfds, &wfds, &xfds, (ret < 0));