vhost: Avoid TX queue when writing directly is faster

Using vhost makes high-volume transfers go nice and fast, especially
we are using 100% of a CPU in the single-threaded OpenConnect process
and just offloading the kernel←→user copies for the tun packets to
the vhost thread instead of having to do them from our single thread
too.

However, for a lightly used link with *occasional* packets, which is
fairly much the definition of a VPN being used for VoIP, it adds a lot
of unwanted latency. If our userspace thread is otherwise going to be
*idle*, and fall back into select() to wait for something else to do,
then we might as well just write the packet *directly* to the tun
device.

So... when the queue is stopped and would need to be kicked, and if
there are only a *few* (heuristic: half max_qlen) packets on the
queue to be sent, just send them directly.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

vhost.c

@@ -326,6 +326,9 @@ static inline int process_ring(struct openconnect_info *vpninfo, int tx, uint64_
 		if (tx) {
 			if (debug_vhost)
 				printf("Free TX packet %p [%d] [used %d]\n", this, ring->seen_used, used_idx);
+			vpninfo->stats.rx_pkts++;
+			vpninfo->stats.rx_bytes += this->len;
+
 			free_pkt(vpninfo, this);
 		} else {
 			if (len < sizeof(this->virtio.h))
@@ -356,6 +359,23 @@ static inline int process_ring(struct openconnect_info *vpninfo, int tx, uint64_
 			this = dequeue_packet(&vpninfo->incoming_queue);
 			if (!this)
 				break;
+
+			/* If only a few packets on the queue, just send them
+			 * directly. The latency is much better. We benefit from
+			 * vhost-net TX when we're overloaded and want to use all
+			 * our CPU on the RX and crypto; there's not a lot of point
+			 * otherwise. */
+			if (!*kick && vpninfo->incoming_queue.count < vpninfo->max_qlen / 2 &&
+			    next_avail == (&ring->used->flags)[2 + (RING_SIZE * 4)]) {
+				if (!os_write_tun(vpninfo, this)) {
+					vpninfo->stats.rx_pkts++;
+					vpninfo->stats.rx_bytes += this->len;
+
+					free_pkt(vpninfo, this);
+					continue;
+				}
+				/* Failed! Pretend it never happened; queue for vhost */
+			}
 			memset(&this->virtio.h, 0, sizeof(this->virtio.h));
 		} else {
 			int len = vpninfo->ip_info.mtu;