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

[wangyunjian]

> -----Original Message-----
> From: Willem de Bruijn [mailto:willemdebruijn.kernel@xxxxxxxxx]
> Sent: Thursday, January 25, 2024 3:05 AM
> To: wangyunjian <wangyunjian@xxxxxxxxxx>; mst@xxxxxxxxxx;
> willemdebruijn.kernel@xxxxxxxxx; jasowang@xxxxxxxxxx; kuba@xxxxxxxxxx;
> davem@xxxxxxxxxxxxx; magnus.karlsson@xxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxxx; xudingke
> <xudingke@xxxxxxxxxx>; wangyunjian <wangyunjian@xxxxxxxxxx>
> Subject: Re: [PATCH net-next 2/2] tun: AF_XDP Rx zero-copy support
> 
> Yunjian Wang 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
> >
> > Signed-off-by: Yunjian Wang <wangyunjian@xxxxxxxxxx>
> 
> I don't fully understand the higher level design of this feature yet.

This feature can reduce the memory copy for the xdp program. 

> 
> But some initial comments at the code level.
> 
> > ---
> >  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);
> 
> Please release state in inverse order of acquire.

Will done.

> 
> >  		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;
> > +	}
> 
> No need for rcu_read_lock with rtnl_dereference.
> 
> Consider ASSERT_RTNL() if unsure whether this patch could be reached without
> the rtnl held.

Will done.

> 
> > +
> > +	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);
> 
> What are the concurrency semantics: there's a spinlock to make the update to
> xdp_rxq and pool a critical section, but the bitmap is not part of this? Please
> also then document why the irqsave.

The bitmap indicates whether the XDP pool is enabled on the current queue,
reducing the impact of the non-enabled XDP pool on the original scenario.
The XDP program may release the pool when receiving packets. So the lock is
only used to protect the pool.

> 
> > +
> > +	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);
> 
> Time of check to time of use race between test and clear? Or is there no race
> because anything that clears will hold the RTNL? If so, please add a comment.
> 
> > +
> > +	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;
> > +}
> > +
> 
> This is almost a copy of tun_put_user_xdp. Can we refactor to avoid duplicating
> code (here and elsewhere this may apply).

Will done.

> 
> >  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);
> 
> For the xsk maintainers if they're reading along: this two line pattern is seen
> quite often. Deserves a helper in a header file?
> 
> > +				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)) {
> > +			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);
> 
> Peek semantics usually don't result in releasing the buffer. Am I
> misunderstanding this operation?
> > +		}
> > +		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
> 
> For virtio maintainer: is it okay to have tun and vhost/net changes in the same
> patch, or is it better to split them?
> 
> > @@ -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++];
> 
> Why this change?
> 
> >  }
> >
> >  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);
> >
> >  	spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
> >  	head = skb_peek(&sk->sk_receive_queue);
> > --
> > 2.33.0
> >
> 

Thanks