notifier: event notifier implementation

event notifiers are slightly generalized eventfd descriptors. Current
implementation depends on eventfd because vhost is the only user, and
vhost depends on eventfd anyway, but a stub is provided for non-eventfd
case.

We'll be able to further generalize this when another user comes along
and we see how to best do this.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

2 files changed, 78 insertions, 0 deletions

diff --git a/hw/event_notifier.c b/hw/event_notifier.c
new file mode 100644
index 0000000000..13f3656460
--- /dev/null
+++ b/hw/event_notifier.c
@@ -0,0 +1,62 @@
+/*
+ * event notifier support
+ *
+ * Copyright Red Hat, Inc. 2010
+ *
+ * Authors:
+ *  Michael S. Tsirkin <mst@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ */
+
+#include "hw.h"
+#include "event_notifier.h"
+#ifdef CONFIG_EVENTFD
+#include <sys/eventfd.h>
+#endif
+
+int event_notifier_init(EventNotifier *e, int active)
+{
+#ifdef CONFIG_EVENTFD
+    int fd = eventfd(!!active, EFD_NONBLOCK | EFD_CLOEXEC);
+    if (fd < 0)
+        return -errno;
+    e->fd = fd;
+    return 0;
+#else
+    return -ENOSYS;
+#endif
+}
+
+void event_notifier_cleanup(EventNotifier *e)
+{
+    close(e->fd);
+}
+
+int event_notifier_get_fd(EventNotifier *e)
+{
+    return e->fd;
+}
+
+int event_notifier_test_and_clear(EventNotifier *e)
+{
+    uint64_t value;
+    int r = read(e->fd, &value, sizeof(value));
+    return r == sizeof(value);
+}
+
+int event_notifier_test(EventNotifier *e)
+{
+    uint64_t value;
+    int r = read(e->fd, &value, sizeof(value));
+    if (r == sizeof(value)) {
+        /* restore previous value. */
+        int s = write(e->fd, &value, sizeof(value));
+        /* never blocks because we use EFD_SEMAPHORE.
+         * If we didn't we'd get EAGAIN on overflow
+         * and we'd have to write code to ignore it. */
+        assert(s == sizeof(value));
+    }
+    return r == sizeof(value);
+}
diff --git a/hw/event_notifier.h b/hw/event_notifier.h
new file mode 100644
index 0000000000..24117ea97b
--- /dev/null
+++ b/hw/event_notifier.h
@@ -0,0 +1,16 @@
+#ifndef QEMU_EVENT_NOTIFIER_H
+#define QEMU_EVENT_NOTIFIER_H
+
+#include "qemu-common.h"
+
+struct EventNotifier {
+	int fd;
+};
+
+int event_notifier_init(EventNotifier *, int active);
+void event_notifier_cleanup(EventNotifier *);
+int event_notifier_get_fd(EventNotifier *);
+int event_notifier_test_and_clear(EventNotifier *);
+int event_notifier_test(EventNotifier *);
+
+#endif