vhost_net: basic in_order support

This patch introduces basic in-order support for vhost-net. By
recording the number of batched buffers in an array when calling
`vhost_add_used_and_signal_n()`, we can reduce the number of userspace
accesses. Note that the vhost-net batching logic is kept as we still
count the number of buffers there.

Testing Results:

With testpmd:

- TX: txonly mode + vhost_net with XDP_DROP on TAP shows a 17.5%
  improvement, from 4.75 Mpps to 5.35 Mpps.
- RX: No obvious improvements were observed.

With virtio-ring in-order experimental code in the guest:

- TX: pktgen in the guest + XDP_DROP on TAP shows a 19% improvement,
  from 5.2 Mpps to 6.2 Mpps.
- RX: pktgen on TAP with vhost_net + XDP_DROP in the guest achieves a
  6.1% improvement, from 3.47 Mpps to 3.61 Mpps.

Acked-by: Jonah Palmer <jonah.palmer@oracle.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20250714084755.11921-4-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>

Author: jasowang

drivers/vhost/net.c

@@ -74,7 +74,8 @@ enum {
 			 (1ULL << VHOST_NET_F_VIRTIO_NET_HDR) |
 			 (1ULL << VIRTIO_NET_F_MRG_RXBUF) |
 			 (1ULL << VIRTIO_F_ACCESS_PLATFORM) |
-			 (1ULL << VIRTIO_F_RING_RESET)
+			 (1ULL << VIRTIO_F_RING_RESET) |
+			 (1ULL << VIRTIO_F_IN_ORDER)
 };
 
 enum {
@@ -450,16 +451,17 @@ static int vhost_net_enable_vq(struct vhost_net *n,
 	return vhost_poll_start(poll, sock->file);
 }
 
-static void vhost_net_signal_used(struct vhost_net_virtqueue *nvq)
+static void vhost_net_signal_used(struct vhost_net_virtqueue *nvq,
+				  unsigned int count)
 {
 	struct vhost_virtqueue *vq = &nvq->vq;
 	struct vhost_dev *dev = vq->dev;
 
 	if (!nvq->done_idx)
 		return;
 
-	vhost_add_used_and_signal_n(dev, vq, vq->heads, NULL,
-				    nvq->done_idx);
+	vhost_add_used_and_signal_n(dev, vq, vq->heads,
+				    vq->nheads, count);
 	nvq->done_idx = 0;
 }
 
@@ -468,13 +470,20 @@ static void vhost_tx_batch(struct vhost_net *net,
 			   struct socket *sock,
 			   struct msghdr *msghdr)
 {
+	struct vhost_virtqueue *vq = &nvq->vq;
+	bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
 	struct tun_msg_ctl ctl = {
 		.type = TUN_MSG_PTR,
 		.num = nvq->batched_xdp,
 		.ptr = nvq->xdp,
 	};
 	int i, err;
 
+	if (in_order) {
+		vq->heads[0].len = 0;
+		vq->nheads[0] = nvq->done_idx;
+	}
+
 	if (nvq->batched_xdp == 0)
 		goto signal_used;
 
@@ -496,7 +505,7 @@ static void vhost_tx_batch(struct vhost_net *net,
 	}
 
 signal_used:
-	vhost_net_signal_used(nvq);
+	vhost_net_signal_used(nvq, in_order ? 1 : nvq->done_idx);
 	nvq->batched_xdp = 0;
 }
 
@@ -758,6 +767,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
 	int sent_pkts = 0;
 	bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX);
 	bool busyloop_intr;
+	bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
 
 	do {
 		busyloop_intr = false;
@@ -794,11 +804,13 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
 				break;
 			}
 
-			/* We can't build XDP buff, go for single
-			 * packet path but let's flush batched
-			 * packets.
-			 */
-			vhost_tx_batch(net, nvq, sock, &msg);
+			if (nvq->batched_xdp) {
+				/* We can't build XDP buff, go for single
+				 * packet path but let's flush batched
+				 * packets.
+				 */
+				vhost_tx_batch(net, nvq, sock, &msg);
+			}
 			msg.msg_control = NULL;
 		} else {
 			if (tx_can_batch(vq, total_len))
@@ -819,8 +831,12 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
 			pr_debug("Truncated TX packet: len %d != %zd\n",
 				 err, len);
 done:
-		vq->heads[nvq->done_idx].id = cpu_to_vhost32(vq, head);
-		vq->heads[nvq->done_idx].len = 0;
+		if (in_order) {
+			vq->heads[0].id = cpu_to_vhost32(vq, head);
+		} else {
+			vq->heads[nvq->done_idx].id = cpu_to_vhost32(vq, head);
+			vq->heads[nvq->done_idx].len = 0;
+		}
 		++nvq->done_idx;
 	} while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
 
@@ -999,7 +1015,7 @@ static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk)
 }
 
 static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
-				      bool *busyloop_intr)
+				      bool *busyloop_intr, unsigned int count)
 {
 	struct vhost_net_virtqueue *rnvq = &net->vqs[VHOST_NET_VQ_RX];
 	struct vhost_net_virtqueue *tnvq = &net->vqs[VHOST_NET_VQ_TX];
@@ -1009,7 +1025,7 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
 
 	if (!len && rvq->busyloop_timeout) {
 		/* Flush batched heads first */
-		vhost_net_signal_used(rnvq);
+		vhost_net_signal_used(rnvq, count);
 		/* Both tx vq and rx socket were polled here */
 		vhost_net_busy_poll(net, rvq, tvq, busyloop_intr, true);
 
@@ -1021,22 +1037,25 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
 
 /* This is a multi-buffer version of vhost_get_desc, that works if
  *	vq has read descriptors only.
- * @vq		- the relevant virtqueue
+ * @nvq		- the relevant vhost_net virtqueue
  * @datalen	- data length we'll be reading
  * @iovcount	- returned count of io vectors we fill
  * @log		- vhost log
  * @log_num	- log offset
  * @quota       - headcount quota, 1 for big buffer
  *	returns number of buffer heads allocated, negative on error
  */
-static int get_rx_bufs(struct vhost_virtqueue *vq,
+static int get_rx_bufs(struct vhost_net_virtqueue *nvq,
 		       struct vring_used_elem *heads,
+		       u16 *nheads,
 		       int datalen,
 		       unsigned *iovcount,
 		       struct vhost_log *log,
 		       unsigned *log_num,
 		       unsigned int quota)
 {
+	struct vhost_virtqueue *vq = &nvq->vq;
+	bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
 	unsigned int out, in;
 	int seg = 0;
 	int headcount = 0;
@@ -1073,14 +1092,16 @@ static int get_rx_bufs(struct vhost_virtqueue *vq,
 			nlogs += *log_num;
 			log += *log_num;
 		}
-		heads[headcount].id = cpu_to_vhost32(vq, d);
 		len = iov_length(vq->iov + seg, in);
-		heads[headcount].len = cpu_to_vhost32(vq, len);
-		datalen -= len;
+		if (!in_order) {
+			heads[headcount].id = cpu_to_vhost32(vq, d);
+			heads[headcount].len = cpu_to_vhost32(vq, len);
+		}
 		++headcount;
+		datalen -= len;
 		seg += in;
 	}
-	heads[headcount - 1].len = cpu_to_vhost32(vq, len + datalen);
+
 	*iovcount = seg;
 	if (unlikely(log))
 		*log_num = nlogs;
@@ -1090,6 +1111,15 @@ static int get_rx_bufs(struct vhost_virtqueue *vq,
 		r = UIO_MAXIOV + 1;
 		goto err;
 	}
+
+	if (!in_order)
+		heads[headcount - 1].len = cpu_to_vhost32(vq, len + datalen);
+	else {
+		heads[0].len = cpu_to_vhost32(vq, len + datalen);
+		heads[0].id = cpu_to_vhost32(vq, d);
+		nheads[0] = headcount;
+	}
+
 	return headcount;
 err:
 	vhost_discard_vq_desc(vq, headcount);
@@ -1102,6 +1132,8 @@ static void handle_rx(struct vhost_net *net)
 {
 	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_RX];
 	struct vhost_virtqueue *vq = &nvq->vq;
+	bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
+	unsigned int count = 0;
 	unsigned in, log;
 	struct vhost_log *vq_log;
 	struct msghdr msg = {
@@ -1149,12 +1181,13 @@ static void handle_rx(struct vhost_net *net)
 
 	do {
 		sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
-						      &busyloop_intr);
+						      &busyloop_intr, count);
 		if (!sock_len)
 			break;
 		sock_len += sock_hlen;
 		vhost_len = sock_len + vhost_hlen;
-		headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
+		headcount = get_rx_bufs(nvq, vq->heads + count,
+					vq->nheads + count,
 					vhost_len, &in, vq_log, &log,
 					likely(mergeable) ? UIO_MAXIOV : 1);
 		/* On error, stop handling until the next kick. */
@@ -1230,8 +1263,11 @@ static void handle_rx(struct vhost_net *net)
 			goto out;
 		}
 		nvq->done_idx += headcount;
-		if (nvq->done_idx > VHOST_NET_BATCH)
-			vhost_net_signal_used(nvq);
+		count += in_order ? 1 : headcount;
+		if (nvq->done_idx > VHOST_NET_BATCH) {
+			vhost_net_signal_used(nvq, count);
+			count = 0;
+		}
 		if (unlikely(vq_log))
 			vhost_log_write(vq, vq_log, log, vhost_len,
 					vq->iov, in);
@@ -1243,7 +1279,7 @@ static void handle_rx(struct vhost_net *net)
 	else if (!sock_len)
 		vhost_net_enable_vq(net, vq);
 out:
-	vhost_net_signal_used(nvq);
+	vhost_net_signal_used(nvq, count);
 	mutex_unlock(&vq->mutex);
 }