tty: hvc: hvc_poll() fix read loop hang

Commit ec97eaad13 ("tty: hvc: hvc_poll() break hv read loop") causes
the virtio console to hang at times (e.g., if you paste a bunch of
characters to it.

The reason is that get_chars must return 0 before we can be sure the
driver will kick or poll input again, but this change only scheduled a
poll if get_chars had returned a full count. Change this to poll on
any > 0 count.

Reported-by: Matteo Croce <mcroce@redhat.com>
Reported-by: Jason Gunthorpe <jgg@mellanox.com>
Tested-by: Matteo Croce <mcroce@redhat.com>
Tested-by: Jason Gunthorpe <jgg@mellanox.com>
Tested-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/hvc/hvc_console.c

@@ -717,10 +717,13 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep)
 #endif /* CONFIG_MAGIC_SYSRQ */
 		tty_insert_flip_char(&hp->port, buf[i], 0);
 	}
-	if (n == count)
-		poll_mask |= HVC_POLL_READ;
 	read_total = n;
 
+	/*
+	 * Latency break, schedule another poll immediately.
+	 */
+	poll_mask |= HVC_POLL_READ;
+
  out:
 	/* Wakeup write queue if necessary */
 	if (hp->do_wakeup) {