net: handle EAGAIN from tapfd write()

If a write() on tapfd returns EAGAIN, return zero so that the packet
gets queued (in the case of async send) and enable polling tapfd for
writing.

When tapfd becomes writable, disable write polling and flush any queued
packets.
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

net.c

@@ -1044,19 +1044,21 @@ typedef struct TAPState {
     char down_script_arg[128];
     uint8_t buf[4096];
     unsigned int read_poll : 1;
+    unsigned int write_poll : 1;
 } TAPState;
 
 static int launch_script(const char *setup_script, const char *ifname, int fd);
 
 static int tap_can_send(void *opaque);
 static void tap_send(void *opaque);
+static void tap_writable(void *opaque);
 
 static void tap_update_fd_handler(TAPState *s)
 {
     qemu_set_fd_handler2(s->fd,
                          s->read_poll  ? tap_can_send : NULL,
                          s->read_poll  ? tap_send     : NULL,
-                         NULL,
+                         s->write_poll ? tap_writable : NULL,
                          s);
 }
 
@@ -1066,6 +1068,21 @@ static void tap_read_poll(TAPState *s, int enable)
     tap_update_fd_handler(s);
 }
 
+static void tap_write_poll(TAPState *s, int enable)
+{
+    s->write_poll = !!enable;
+    tap_update_fd_handler(s);
+}
+
+static void tap_writable(void *opaque)
+{
+    TAPState *s = opaque;
+
+    tap_write_poll(s, 0);
+
+    qemu_flush_queued_packets(s->vc);
+}
+
 static ssize_t tap_receive_iov(VLANClientState *vc, const struct iovec *iov,
                                int iovcnt)
 {
@@ -1074,7 +1091,12 @@ static ssize_t tap_receive_iov(VLANClientState *vc, const struct iovec *iov,
 
     do {
         len = writev(s->fd, iov, iovcnt);
-    } while (len == -1 && (errno == EINTR || errno == EAGAIN));
+    } while (len == -1 && errno == EINTR);
+
+    if (len == -1 && errno == EAGAIN) {
+        tap_write_poll(s, 1);
+        return 0;
+    }
 
     return len;
 }
@@ -1150,6 +1172,7 @@ static void tap_cleanup(VLANClientState *vc)
         launch_script(s->down_script, s->down_script_arg, s->fd);
 
     tap_read_poll(s, 0);
+    tap_write_poll(s, 0);
     close(s->fd);
     qemu_free(s);
 }