dataplane: add event loop

Outside the safety of the global mutex we need to poll on file
descriptors.  I found epoll(2) is a convenient way to do that, although
other options could replace this module in the future (such as an
AioContext-based loop or glib's GMainLoop).

One important feature of this small event loop implementation is that
the loop can be terminated in a thread-safe way.  This allows QEMU to
stop the data plane thread cleanly.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/dataplane/Makefile.objs

 ifeq ($(CONFIG_VIRTIO), y)
-common-obj-$(CONFIG_VIRTIO_BLK_DATA_PLANE) += hostmem.o vring.o
+common-obj-$(CONFIG_VIRTIO_BLK_DATA_PLANE) += hostmem.o vring.o event-poll.o
 endif

hw/dataplane/event-poll.c

+/*
+ * Event loop with file descriptor polling
+ *
+ * Copyright 2012 IBM, Corp.
+ * Copyright 2012 Red Hat, Inc. and/or its affiliates
+ *
+ * Authors:
+ *   Stefan Hajnoczi <stefanha@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#include <sys/epoll.h>
+#include "hw/dataplane/event-poll.h"
+
+/* Add an event notifier and its callback for polling */
+void event_poll_add(EventPoll *poll, EventHandler *handler,
+                    EventNotifier *notifier, EventCallback *callback)
+{
+    struct epoll_event event = {
+        .events = EPOLLIN,
+        .data.ptr = handler,
+    };
+    handler->notifier = notifier;
+    handler->callback = callback;
+    if (epoll_ctl(poll->epoll_fd, EPOLL_CTL_ADD,
+                  event_notifier_get_fd(notifier), &event) != 0) {
+        fprintf(stderr, "failed to add event handler to epoll: %m\n");
+        exit(1);
+    }
+}
+
+/* Event callback for stopping event_poll() */
+static void handle_stop(EventHandler *handler)
+{
+    /* Do nothing */
+}
+
+void event_poll_init(EventPoll *poll)
+{
+    /* Create epoll file descriptor */
+    poll->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
+    if (poll->epoll_fd < 0) {
+        fprintf(stderr, "epoll_create1 failed: %m\n");
+        exit(1);
+    }
+
+    /* Set up stop notifier */
+    if (event_notifier_init(&poll->stop_notifier, 0) < 0) {
+        fprintf(stderr, "failed to init stop notifier\n");
+        exit(1);
+    }
+    event_poll_add(poll, &poll->stop_handler,
+                   &poll->stop_notifier, handle_stop);
+}
+
+void event_poll_cleanup(EventPoll *poll)
+{
+    event_notifier_cleanup(&poll->stop_notifier);
+    close(poll->epoll_fd);
+    poll->epoll_fd = -1;
+}
+
+/* Block until the next event and invoke its callback */
+void event_poll(EventPoll *poll)
+{
+    EventHandler *handler;
+    struct epoll_event event;
+    int nevents;
+
+    /* Wait for the next event.  Only do one event per call to keep the
+     * function simple, this could be changed later. */
+    do {
+        nevents = epoll_wait(poll->epoll_fd, &event, 1, -1);
+    } while (nevents < 0 && errno == EINTR);
+    if (unlikely(nevents != 1)) {
+        fprintf(stderr, "epoll_wait failed: %m\n");
+        exit(1); /* should never happen */
+    }
+
+    /* Find out which event handler has become active */
+    handler = event.data.ptr;
+
+    /* Clear the eventfd */
+    event_notifier_test_and_clear(handler->notifier);
+
+    /* Handle the event */
+    handler->callback(handler);
+}
+
+/* Stop event_poll()
+ *
+ * This function can be used from another thread.
+ */
+void event_poll_notify(EventPoll *poll)
+{
+    event_notifier_set(&poll->stop_notifier);
+}

hw/dataplane/event-poll.h

+/*
+ * Event loop with file descriptor polling
+ *
+ * Copyright 2012 IBM, Corp.
+ * Copyright 2012 Red Hat, Inc. and/or its affiliates
+ *
+ * Authors:
+ *   Stefan Hajnoczi <stefanha@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#ifndef EVENT_POLL_H
+#define EVENT_POLL_H
+
+#include "qemu/event_notifier.h"
+
+typedef struct EventHandler EventHandler;
+typedef void EventCallback(EventHandler *handler);
+struct EventHandler {
+    EventNotifier *notifier;        /* eventfd */
+    EventCallback *callback;        /* callback function */
+};
+
+typedef struct {
+    int epoll_fd;                   /* epoll(2) file descriptor */
+    EventNotifier stop_notifier;    /* stop poll notifier */
+    EventHandler stop_handler;      /* stop poll handler */
+} EventPoll;
+
+void event_poll_add(EventPoll *poll, EventHandler *handler,
+                    EventNotifier *notifier, EventCallback *callback);
+void event_poll_init(EventPoll *poll);
+void event_poll_cleanup(EventPoll *poll);
+void event_poll(EventPoll *poll);
+void event_poll_notify(EventPoll *poll);
+
+#endif /* EVENT_POLL_H */