Re: [PATCH net-next 2/2] tun: AF_XDP Rx zero-copy support

From: Jason Wang
Date: Wed Jan 24 2024 - 23:49:16 EST

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In reply to: wangyunjian: "RE: [PATCH net-next 2/2] tun: AF_XDP Rx zero-copy support"
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On Wed, Jan 24, 2024 at 5:38 PM Yunjian Wang <wangyunjian@xxxxxxxxxx> wrote:
>
> Now the zero-copy feature of AF_XDP socket is supported by some
> drivers, which can reduce CPU utilization on the xdp program.
> This patch set allows tun to support AF_XDP Rx zero-copy feature.
>
> This patch tries to address this by:
> - Use peek_len to consume a xsk->desc and get xsk->desc length.
> - When the tun support AF_XDP Rx zero-copy, the vq's array maybe empty.
> So add a check for empty vq's array in vhost_net_buf_produce().
> - add XDP_SETUP_XSK_POOL and ndo_xsk_wakeup callback support
> - add tun_put_user_desc function to copy the Rx data to VM

Code explains themselves, let's explain why you need to do this.

1) why you want to use peek_len
2) for "vq's array", what does it mean?
3) from the view of TUN/TAP tun_put_user_desc() is the TX path, so I
guess you meant TX zerocopy instead of RX (as I don't see codes for
RX?)

A big question is how could you handle GSO packets from userspace/guests?

>
> Signed-off-by: Yunjian Wang <wangyunjian@xxxxxxxxxx>
> ---
> drivers/net/tun.c | 165 +++++++++++++++++++++++++++++++++++++++++++-
> drivers/vhost/net.c | 18 +++--
> 2 files changed, 176 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index afa5497f7c35..248b0f8e07d1 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -77,6 +77,7 @@
> #include <net/ax25.h>
> #include <net/rose.h>
> #include <net/6lowpan.h>
> +#include <net/xdp_sock_drv.h>
>
> #include <linux/uaccess.h>
> #include <linux/proc_fs.h>
> @@ -145,6 +146,10 @@ struct tun_file {
> struct tun_struct *detached;
> struct ptr_ring tx_ring;
> struct xdp_rxq_info xdp_rxq;
> + struct xdp_desc desc;
> + /* protects xsk pool */
> + spinlock_t pool_lock;
> + struct xsk_buff_pool *pool;
> };
>
> struct tun_page {
> @@ -208,6 +213,8 @@ struct tun_struct {
> struct bpf_prog __rcu *xdp_prog;
> struct tun_prog __rcu *steering_prog;
> struct tun_prog __rcu *filter_prog;
> + /* tracks AF_XDP ZC enabled queues */
> + unsigned long *af_xdp_zc_qps;
> struct ethtool_link_ksettings link_ksettings;
> /* init args */
> struct file *file;
> @@ -795,6 +802,8 @@ static int tun_attach(struct tun_struct *tun, struct file *file,
>
> tfile->queue_index = tun->numqueues;
> tfile->socket.sk->sk_shutdown &= ~RCV_SHUTDOWN;
> + tfile->desc.len = 0;
> + tfile->pool = NULL;
>
> if (tfile->detached) {
> /* Re-attach detached tfile, updating XDP queue_index */
> @@ -989,6 +998,13 @@ static int tun_net_init(struct net_device *dev)
> return err;
> }
>
> + tun->af_xdp_zc_qps = bitmap_zalloc(MAX_TAP_QUEUES, GFP_KERNEL);
> + if (!tun->af_xdp_zc_qps) {
> + security_tun_dev_free_security(tun->security);
> + free_percpu(dev->tstats);
> + return -ENOMEM;
> + }
> +
> tun_flow_init(tun);
>
> dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST |
> @@ -1009,6 +1025,7 @@ static int tun_net_init(struct net_device *dev)
> tun_flow_uninit(tun);
> security_tun_dev_free_security(tun->security);
> free_percpu(dev->tstats);
> + bitmap_free(tun->af_xdp_zc_qps);
> return err;
> }
> return 0;
> @@ -1222,11 +1239,77 @@ static int tun_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> return 0;
> }
>
> +static int tun_xsk_pool_enable(struct net_device *netdev,
> + struct xsk_buff_pool *pool,
> + u16 qid)
> +{
> + struct tun_struct *tun = netdev_priv(netdev);
> + struct tun_file *tfile;
> + unsigned long flags;
> +
> + rcu_read_lock();
> + tfile = rtnl_dereference(tun->tfiles[qid]);
> + if (!tfile) {
> + rcu_read_unlock();
> + return -ENODEV;
> + }
> +
> + spin_lock_irqsave(&tfile->pool_lock, flags);
> + xsk_pool_set_rxq_info(pool, &tfile->xdp_rxq);
> + tfile->pool = pool;
> + spin_unlock_irqrestore(&tfile->pool_lock, flags);
> +
> + rcu_read_unlock();
> + set_bit(qid, tun->af_xdp_zc_qps);
> +
> + return 0;
> +}
> +
> +static int tun_xsk_pool_disable(struct net_device *netdev, u16 qid)
> +{
> + struct tun_struct *tun = netdev_priv(netdev);
> + struct tun_file *tfile;
> + unsigned long flags;
> +
> + if (!test_bit(qid, tun->af_xdp_zc_qps))
> + return 0;
> +
> + clear_bit(qid, tun->af_xdp_zc_qps);
> +
> + rcu_read_lock();
> + tfile = rtnl_dereference(tun->tfiles[qid]);
> + if (!tfile) {
> + rcu_read_unlock();
> + return 0;
> + }
> +
> + spin_lock_irqsave(&tfile->pool_lock, flags);
> + if (tfile->desc.len) {
> + xsk_tx_completed(tfile->pool, 1);
> + tfile->desc.len = 0;
> + }
> + tfile->pool = NULL;
> + spin_unlock_irqrestore(&tfile->pool_lock, flags);
> +
> + rcu_read_unlock();
> + return 0;
> +}
> +
> +int tun_xsk_pool_setup(struct net_device *dev, struct xsk_buff_pool *pool,
> + u16 qid)
> +{
> + return pool ? tun_xsk_pool_enable(dev, pool, qid) :
> + tun_xsk_pool_disable(dev, qid);
> +}
> +
> static int tun_xdp(struct net_device *dev, struct netdev_bpf *xdp)
> {
> switch (xdp->command) {
> case XDP_SETUP_PROG:
> return tun_xdp_set(dev, xdp->prog, xdp->extack);
> + case XDP_SETUP_XSK_POOL:
> + return tun_xsk_pool_setup(dev, xdp->xsk.pool,
> + xdp->xsk.queue_id);
> default:
> return -EINVAL;
> }
> @@ -1331,6 +1414,19 @@ static int tun_xdp_tx(struct net_device *dev, struct xdp_buff *xdp)
> return nxmit;
> }
>
> +static int tun_xsk_wakeup(struct net_device *dev, u32 qid, u32 flags)
> +{
> + struct tun_struct *tun = netdev_priv(dev);
> + struct tun_file *tfile;
> +
> + rcu_read_lock();
> + tfile = rcu_dereference(tun->tfiles[qid]);
> + if (tfile)
> + __tun_xdp_flush_tfile(tfile);
> + rcu_read_unlock();
> + return 0;
> +}
> +
> static const struct net_device_ops tap_netdev_ops = {
> .ndo_init = tun_net_init,
> .ndo_uninit = tun_net_uninit,
> @@ -1347,6 +1443,7 @@ static const struct net_device_ops tap_netdev_ops = {
> .ndo_get_stats64 = dev_get_tstats64,
> .ndo_bpf = tun_xdp,
> .ndo_xdp_xmit = tun_xdp_xmit,
> + .ndo_xsk_wakeup = tun_xsk_wakeup,
> .ndo_change_carrier = tun_net_change_carrier,
> };
>
> @@ -1404,7 +1501,8 @@ static void tun_net_initialize(struct net_device *dev)
> /* Currently tun does not support XDP, only tap does. */
> dev->xdp_features = NETDEV_XDP_ACT_BASIC |
> NETDEV_XDP_ACT_REDIRECT |
> - NETDEV_XDP_ACT_NDO_XMIT;
> + NETDEV_XDP_ACT_NDO_XMIT |
> + NETDEV_XDP_ACT_XSK_ZEROCOPY;
>
> break;
> }
> @@ -2213,6 +2311,37 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
> return ptr;
> }
>
> +static ssize_t tun_put_user_desc(struct tun_struct *tun,
> + struct tun_file *tfile,
> + struct xdp_desc *desc,
> + struct iov_iter *iter)
> +{
> + size_t size = desc->len;
> + int vnet_hdr_sz = 0;
> + size_t ret;
> +
> + if (tun->flags & IFF_VNET_HDR) {
> + struct virtio_net_hdr_mrg_rxbuf gso = { 0 };
> +
> + vnet_hdr_sz = READ_ONCE(tun->vnet_hdr_sz);
> + if (unlikely(iov_iter_count(iter) < vnet_hdr_sz))
> + return -EINVAL;
> + if (unlikely(copy_to_iter(&gso, sizeof(gso), iter) !=
> + sizeof(gso)))
> + return -EFAULT;
> + iov_iter_advance(iter, vnet_hdr_sz - sizeof(gso));
> + }
> +
> + ret = copy_to_iter(xsk_buff_raw_get_data(tfile->pool, desc->addr),
> + size, iter) + vnet_hdr_sz;
> +
> + preempt_disable();
> + dev_sw_netstats_tx_add(tun->dev, 1, ret);
> + preempt_enable();
> +
> + return ret;
> +}
> +
> static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
> struct iov_iter *to,
> int noblock, void *ptr)
> @@ -2226,6 +2355,22 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
> }
>
> if (!ptr) {
> + /* Read frames from xsk's desc */
> + if (test_bit(tfile->queue_index, tun->af_xdp_zc_qps)) {
> + spin_lock(&tfile->pool_lock);
> + if (tfile->pool) {
> + ret = tun_put_user_desc(tun, tfile, &tfile->desc, to);
> + xsk_tx_completed(tfile->pool, 1);
> + if (xsk_uses_need_wakeup(tfile->pool))
> + xsk_set_tx_need_wakeup(tfile->pool);
> + tfile->desc.len = 0;
> + } else {
> + ret = -EBADFD;
> + }
> + spin_unlock(&tfile->pool_lock);
> + return ret;
> + }
> +
> /* Read frames from ring */
> ptr = tun_ring_recv(tfile, noblock, &err);
> if (!ptr)
> @@ -2311,6 +2456,7 @@ static void tun_free_netdev(struct net_device *dev)
>
> BUG_ON(!(list_empty(&tun->disabled)));
>
> + bitmap_free(tun->af_xdp_zc_qps);
> free_percpu(dev->tstats);
> tun_flow_uninit(tun);
> security_tun_dev_free_security(tun->security);
> @@ -2666,7 +2812,19 @@ static int tun_peek_len(struct socket *sock)
> if (!tun)
> return 0;
>
> - ret = PTR_RING_PEEK_CALL(&tfile->tx_ring, tun_ptr_peek_len);
> + if (test_bit(tfile->queue_index, tun->af_xdp_zc_qps)) {
> + spin_lock(&tfile->pool_lock);
> + if (tfile->pool && xsk_tx_peek_desc(tfile->pool, &tfile->desc)) {

Does it mean if userspace doesn't peek, we can't read anything?

We need to make sure syscall read works as well as vhost_net.

> + xsk_tx_release(tfile->pool);
> + ret = tfile->desc.len;
> + /* The length of desc must be greater than 0 */
> + if (!ret)
> + xsk_tx_completed(tfile->pool, 1);
> + }
> + spin_unlock(&tfile->pool_lock);
> + } else {
> + ret = PTR_RING_PEEK_CALL(&tfile->tx_ring, tun_ptr_peek_len);
> + }
> tun_put(tun);
>
> return ret;
> @@ -3469,8 +3627,11 @@ static int tun_chr_open(struct inode *inode, struct file * file)
>
> mutex_init(&tfile->napi_mutex);
> RCU_INIT_POINTER(tfile->tun, NULL);
> + spin_lock_init(&tfile->pool_lock);
> tfile->flags = 0;
> tfile->ifindex = 0;
> + tfile->pool = NULL;
> + tfile->desc.len = 0;
>
> init_waitqueue_head(&tfile->socket.wq.wait);
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index f2ed7167c848..a1f143ad2341 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -169,9 +169,10 @@ static int vhost_net_buf_is_empty(struct vhost_net_buf *rxq)
>
> static void *vhost_net_buf_consume(struct vhost_net_buf *rxq)
> {
> - void *ret = vhost_net_buf_get_ptr(rxq);
> - ++rxq->head;
> - return ret;
> + if (rxq->tail == rxq->head)
> + return NULL;
> +
> + return rxq->queue[rxq->head++];
> }
>
> static int vhost_net_buf_produce(struct vhost_net_virtqueue *nvq)
> @@ -993,12 +994,19 @@ static void handle_tx(struct vhost_net *net)
>
> static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk)
> {
> + struct socket *sock = sk->sk_socket;
> struct sk_buff *head;
> int len = 0;
> unsigned long flags;
>
> - if (rvq->rx_ring)
> - return vhost_net_buf_peek(rvq);
> + if (rvq->rx_ring) {
> + len = vhost_net_buf_peek(rvq);
> + if (likely(len))
> + return len;
> + }
> +
> + if (sock->ops->peek_len)
> + return sock->ops->peek_len(sock);

What prevents you from reusing the ptr_ring here? Then you don't need
the above tricks.

Thanks

>
> spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
> head = skb_peek(&sk->sk_receive_queue);
> --
> 2.33.0
>

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