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Message-ID: <57a0dfda-3f15-4056-9434-35d07d3e0c27@oracle.com>
Date: Wed, 9 Jul 2025 12:38:51 -0400
From: Jonah Palmer <jonah.palmer@...cle.com>
To: Jason Wang <jasowang@...hat.com>, mst@...hat.com, eperezma@...hat.com
Cc: kvm@...r.kernel.org, virtualization@...ts.linux.dev,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/2] vhost_net: basic in_order support
On 7/8/25 2:48 AM, Jason Wang wrote:
> 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@...cle.com>
>
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
> drivers/vhost/net.c | 86 ++++++++++++++++++++++++++++++++-------------
> 1 file changed, 61 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 4f9c67f17b49..8ac994b3228a 100644
> --- a/drivers/vhost/net.c
> +++ b/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 {
> @@ -450,7 +451,8 @@ 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;
> @@ -458,8 +460,8 @@ static void vhost_net_signal_used(struct vhost_net_virtqueue *nvq)
> 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;
> }
>
> @@ -468,6 +470,8 @@ 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,
> @@ -475,6 +479,11 @@ static void vhost_tx_batch(struct vhost_net *net,
> };
> 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);
>
> @@ -1021,7 +1037,7 @@ 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
> @@ -1029,14 +1045,17 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
> * @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);
> }
>
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