Poll main loop after I/O events were received

Polling until select returns empty fdsets helps to reduce the switches
between iothread and vcpus. The benefit of this patch is best visible
when running an SMP guest on an SMP host in emulation mode.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

sysemu.h

@@ -67,7 +67,7 @@ void do_info_snapshots(Monitor *mon);
 
 void qemu_announce_self(void);
 
-void main_loop_wait(int nonblocking);
+int main_loop_wait(int nonblocking);
 
 bool qemu_savevm_state_blocked(Monitor *mon);
 int qemu_savevm_state_begin(Monitor *mon, QEMUFile *f, int blk_enable,

vl.c

@@ -1321,7 +1321,7 @@ void qemu_system_vmstop_request(int reason)
     qemu_notify_event();
 }
 
-void main_loop_wait(int nonblocking)
+int main_loop_wait(int nonblocking)
 {
     fd_set rfds, wfds, xfds;
     int ret, nfds;
@@ -1368,6 +1368,7 @@ void main_loop_wait(int nonblocking)
        them.  */
     qemu_bh_poll();
 
+    return ret;
 }
 
 #ifndef CONFIG_IOTHREAD
@@ -1385,7 +1386,8 @@ qemu_irq qemu_system_powerdown;
 
 static void main_loop(void)
 {
-    bool nonblocking = false;
+    bool nonblocking;
+    int last_io __attribute__ ((unused)) = 0;
 #ifdef CONFIG_PROFILER
     int64_t ti;
 #endif
@@ -1394,7 +1396,9 @@ static void main_loop(void)
     qemu_main_loop_start();
 
     for (;;) {
-#ifndef CONFIG_IOTHREAD
+#ifdef CONFIG_IOTHREAD
+        nonblocking = !kvm_enabled() && last_io > 0;
+#else
         nonblocking = cpu_exec_all();
         if (vm_request_pending()) {
             nonblocking = true;
@@ -1403,7 +1407,7 @@ static void main_loop(void)
 #ifdef CONFIG_PROFILER
         ti = profile_getclock();
 #endif
-        main_loop_wait(nonblocking);
+        last_io = main_loop_wait(nonblocking);
 #ifdef CONFIG_PROFILER
         dev_time += profile_getclock() - ti;
 #endif