use eventfd for iothread

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

osdep.c

@@ -37,6 +37,10 @@
 #include <sys/statvfs.h>
 #endif
 
+#ifdef CONFIG_EVENTFD
+#include <sys/eventfd.h>
+#endif
+
 #ifdef _WIN32
 #include <windows.h>
 #elif defined(CONFIG_BSD)
@@ -280,6 +284,34 @@ ssize_t qemu_write_full(int fd, const void *buf, size_t count)
 }
 
 #ifndef _WIN32
+/*
+ * Creates an eventfd that looks like a pipe and has EFD_CLOEXEC set.
+ */
+int qemu_eventfd(int fds[2])
+{
+    int ret;
+
+#ifdef CONFIG_EVENTFD
+    ret = eventfd(0, 0);
+    if (ret >= 0) {
+        fds[0] = ret;
+        qemu_set_cloexec(ret);
+        if ((fds[1] = dup(ret)) == -1) {
+            close(ret);
+            return -1;
+        }
+        qemu_set_cloexec(fds[1]);
+        return 0;
+    }
+
+    if (errno != ENOSYS) {
+        return -1;
+    }
+#endif
+
+    return qemu_pipe(fds);
+}
+
 /*
  * Creates a pipe with FD_CLOEXEC set on both file descriptors
  */

qemu-common.h

@@ -170,6 +170,7 @@ ssize_t qemu_write_full(int fd, const void *buf, size_t count)
 void qemu_set_cloexec(int fd);
 
 #ifndef _WIN32
+int qemu_eventfd(int pipefd[2]);
 int qemu_pipe(int pipefd[2]);
 #endif
 

vl.c

@@ -3211,14 +3211,15 @@ static int io_thread_fd = -1;
 
 static void qemu_event_increment(void)
 {
-    static const char byte = 0;
+    /* Write 8 bytes to be compatible with eventfd.  */
+    static uint64_t val = 1;
     ssize_t ret;
 
     if (io_thread_fd == -1)
         return;
 
     do {
-        ret = write(io_thread_fd, &byte, sizeof(byte));
+        ret = write(io_thread_fd, &val, sizeof(val));
     } while (ret < 0 && errno == EINTR);
 
     /* EAGAIN is fine, a read must be pending.  */
@@ -3235,7 +3236,7 @@ static void qemu_event_read(void *opaque)
     ssize_t len;
     char buffer[512];
 
-    /* Drain the notify pipe */
+    /* Drain the notify pipe.  For eventfd, only 8 bytes will be read.  */
     do {
         len = read(fd, buffer, sizeof(buffer));
     } while ((len == -1 && errno == EINTR) || len == sizeof(buffer));
@@ -3246,7 +3247,7 @@ static int qemu_event_init(void)
     int err;
     int fds[2];
 
-    err = qemu_pipe(fds);
+    err = qemu_eventfd(fds);
     if (err == -1)
         return -errno;