This avoids a wakeup being lost, in case the iothread handles the
signal before select().

Copied and possibly damaged from kvm-userspace.

Index: qemu/vl.c
===================================================================
--- qemu.orig/vl.c
+++ qemu/vl.c
@@ -155,8 +155,6 @@
 //#define DEBUG_NET
 //#define DEBUG_SLIRP
 
-#define SIG_IPI (SIGRTMIN+4)
-
 #ifdef DEBUG_IOPORT
 #  define LOG_IOPORT(...) qemu_log_mask(CPU_LOG_IOPORT, ## __VA_ARGS__)
 #else
@@ -273,6 +271,8 @@ QemuThread cpus_thread;
 
 QemuCond qemu_pause_cond;
 
+static int io_thread_fd = -1;
+
 static void pause_all_vcpus(void);
 static void resume_all_vcpus(void);
 
@@ -3723,7 +3723,6 @@ static void block_io_signals(void)
     sigaddset(&set, SIGUSR2);
     sigaddset(&set, SIGIO);
     sigaddset(&set, SIGALRM);
-    sigaddset(&set, SIG_IPI);
     pthread_sigmask(SIG_BLOCK, &set, NULL);
 
     sigemptyset(&set);
@@ -3735,30 +3734,19 @@ static void block_io_signals(void)
     sigaction(SIGUSR1, &sigact, NULL);
 }
 
-/* used to wake up the io thread */
-static void sig_ipi_handler(int sig)
-{
-}
-
 static void unblock_io_signals(void)
 {
     sigset_t set;
-    struct sigaction sigact;
 
     sigemptyset(&set);
     sigaddset(&set, SIGUSR2);
     sigaddset(&set, SIGIO);
     sigaddset(&set, SIGALRM);
-    sigaddset(&set, SIG_IPI);
     pthread_sigmask(SIG_UNBLOCK, &set, NULL);
 
     sigemptyset(&set);
     sigaddset(&set, SIGUSR1);
     pthread_sigmask(SIG_BLOCK, &set, NULL);
-
-    memset(&sigact, 0, sizeof(sigact));
-    sigact.sa_handler = sig_ipi_handler;
-    sigaction(SIG_IPI, &sigact, NULL);
 }
 
 int qemu_notify_event(void *cpu_env)
@@ -3774,11 +3762,51 @@ int qemu_notify_event(void *cpu_env)
 
 void main_loop_break(void)
 {
-    QemuThread me;
-    qemu_thread_self(&me);
-    if (qemu_thread_equal(&io_thread, &me))
+    uint64_t value = 1;
+    char buffer[8];
+    size_t offset = 0;
+
+    if (io_thread_fd == -1)
         return;
-    qemu_thread_signal(&io_thread, SIG_IPI);
+
+    memcpy(buffer, &value, sizeof(value));
+
+    while (offset < 8) {
+        ssize_t len;
+
+        len = write(io_thread_fd, buffer + offset, 8 - offset);
+        if (len == -1 && errno == EINTR)
+            continue;
+
+        if (len <= 0)
+            break;
+
+        offset += len;
+    }
+
+    if (offset != 8)
+        fprintf(stderr, "failed to notify io thread\n");
+}
+
+/* Used to break IO thread out of select */
+static void io_thread_wakeup(void *opaque)
+{
+    int fd = (unsigned long)opaque;
+    char buffer[8];
+    size_t offset = 0;
+
+    while (offset < 8) {
+        ssize_t len;
+
+        len = read(fd, buffer + offset, 8 - offset);
+        if (len == -1 && errno == EINTR)
+            continue;
+
+        if (len <= 0)
+            break;
+
+        offset += len;
+    }
 }
 
 static void qemu_signal_lock(unsigned int msecs)
@@ -4086,6 +4114,20 @@ static void resume_all_vcpus(void)
     }
 }
 
+static void setup_iothread_fd(void)
+{
+    int fds[2];
+
+    if (pipe(fds) == -1) {
+        fprintf(stderr, "failed to create iothread pipe");
+        exit(0);
+    }
+
+    qemu_set_fd_handler2(fds[0], NULL, io_thread_wakeup, NULL,
+                         (void *)(unsigned long)fds[0]);
+    io_thread_fd = fds[1];
+}
+
 static void main_loop(void)
 {
     int r;
@@ -4096,6 +4138,7 @@ static void main_loop(void)
     qemu_cond_init(&qemu_pause_cond);
 
     qemu_thread_self(&io_thread);
+    setup_iothread_fd();
 
     unblock_io_signals();
 

--