virtio: console: add port stats for bytes received, sent and discarded

This commit adds port-specific stats for the number of bytes received,
sent and discarded.  They're exposed via the debugfs interface.  This
data can be used to check for data loss bugs (or disprove such claims).
It can also be used for accounting, if there's such a need.

The stats remain valid throughout the lifetime of the port.  Unplugging
a port will reset the stats.  The numbers are not reset across port
opens/closes.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -152,6 +152,10 @@ struct ports_device {
 	int chr_major;
 };
 
+struct port_stats {
+	unsigned long bytes_sent, bytes_received, bytes_discarded;
+};
+
 /* This struct holds the per-port data */
 struct port {
 	/* Next port in the list, head is in the ports_device */
@@ -179,6 +183,13 @@ struct port {
 	/* File in the debugfs directory that exposes this port's information */
 	struct dentry *debugfs_file;
 
+	/*
+	 * Keep count of the bytes sent, received and discarded for
+	 * this port for accounting and debugging purposes.  These
+	 * counts are not reset across port open / close events.
+	 */
+	struct port_stats stats;
+
 	/*
 	 * The entries in this struct will be valid if this port is
 	 * hooked up to an hvc console
@@ -360,6 +371,7 @@ static struct port_buffer *get_inbuf(struct port *port)
 	if (buf) {
 		buf->len = len;
 		buf->offset = 0;
+		port->stats.bytes_received += len;
 	}
 	return buf;
 }
@@ -396,6 +408,7 @@ static void discard_port_data(struct port *port)
 
 	err = 0;
 	while (buf) {
+		port->stats.bytes_discarded += buf->len - buf->offset;
 		if (add_inbuf(port->in_vq, buf) < 0) {
 			err++;
 			free_buf(buf);
@@ -519,6 +532,8 @@ static ssize_t send_buf(struct port *port, void *in_buf, size_t in_count,
 		cpu_relax();
 done:
 	spin_unlock_irqrestore(&port->outvq_lock, flags);
+
+	port->stats.bytes_sent += in_count;
 	/*
 	 * We're expected to return the amount of data we wrote -- all
 	 * of it
@@ -1048,6 +1063,14 @@ static ssize_t debugfs_read(struct file *filp, char __user *ubuf,
 			       "host_connected: %d\n", port->host_connected);
 	out_offset += snprintf(buf + out_offset, out_count - out_offset,
 			       "outvq_full: %d\n", port->outvq_full);
+	out_offset += snprintf(buf + out_offset, out_count - out_offset,
+			       "bytes_sent: %lu\n", port->stats.bytes_sent);
+	out_offset += snprintf(buf + out_offset, out_count - out_offset,
+			       "bytes_received: %lu\n",
+			       port->stats.bytes_received);
+	out_offset += snprintf(buf + out_offset, out_count - out_offset,
+			       "bytes_discarded: %lu\n",
+			       port->stats.bytes_discarded);
 	out_offset += snprintf(buf + out_offset, out_count - out_offset,
 			       "is_console: %s\n",
 			       is_console_port(port) ? "yes" : "no");
@@ -1133,6 +1156,7 @@ static int add_port(struct ports_device *portdev, u32 id)
 	port->cons.ws.ws_row = port->cons.ws.ws_col = 0;
 
 	port->host_connected = port->guest_connected = false;
+	port->stats = (struct port_stats) { 0 };
 
 	port->outvq_full = false;