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>

Makefile.target

@@ -165,6 +165,7 @@ obj-y = arch_init.o cpus.o monitor.o machine.o gdbstub.o vl.o
 # virtio has to be here due to weird dependency between PCI and virtio-net.
 # need to fix this properly
 obj-y += virtio-blk.o virtio-balloon.o virtio-net.o virtio-pci.o virtio-serial-bus.o
+obj-y += event_notifier.o
 obj-y += rwhandler.o
 obj-$(CONFIG_KVM) += kvm.o kvm-all.o
 LIBS+=-lz

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);
+}

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

qemu-common.h

@@ -228,6 +228,7 @@ typedef struct uWireSlave uWireSlave;
 typedef struct I2SCodec I2SCodec;
 typedef struct DeviceState DeviceState;
 typedef struct SSIBus SSIBus;
+typedef struct EventNotifier EventNotifier;
 
 typedef uint64_t pcibus_t;