Re: [PATCH net-next v2 5/7] virtio-net, xsk: realize the function of xsk packet sending

[Jason Wang <jasowang@redhat.com>]

On 2021/1/16 上午10:59, Xuan Zhuo wrote:
> virtnet_xsk_run will be called in the tx interrupt handling function
> virtnet_poll_tx.
>
> The sending process gets desc from the xsk tx queue, and assembles it to
> send the data.
>
> Compared with other drivers, a special place is that the page of the
> data in xsk is used here instead of the dma address. Because the virtio
> interface does not use the dma address.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>   drivers/net/virtio_net.c | 200 ++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 197 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index a62d456..42aa9ad 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -119,6 +119,8 @@ struct virtnet_xsk_hdr {
>   	u32 len;
>   };
>   
> +#define VIRTNET_STATE_XSK_WAKEUP 1
> +
>   #define VIRTNET_SQ_STAT(m)	offsetof(struct virtnet_sq_stats, m)
>   #define VIRTNET_RQ_STAT(m)	offsetof(struct virtnet_rq_stats, m)
>   
> @@ -163,9 +165,12 @@ struct send_queue {
>   		struct xsk_buff_pool   __rcu *pool;
>   		struct virtnet_xsk_hdr __rcu *hdr;
>   
> +		unsigned long          state;
>   		u64                    hdr_con;
>   		u64                    hdr_pro;
>   		u64                    hdr_n;
> +		struct xdp_desc        last_desc;
> +		bool                   wait_slot;
>   	} xsk;
>   };
>   
> @@ -284,6 +289,8 @@ static void __free_old_xmit_ptr(struct send_queue *sq, bool in_napi,
>   				bool xsk_wakeup,
>   				unsigned int *_packets, unsigned int *_bytes);
>   static void free_old_xmit_skbs(struct send_queue *sq, bool in_napi);
> +static int virtnet_xsk_run(struct send_queue *sq,
> +			   struct xsk_buff_pool *pool, int budget);
>   
>   static bool is_xdp_frame(void *ptr)
>   {
> @@ -1590,6 +1597,8 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
>   	struct virtnet_info *vi = sq->vq->vdev->priv;
>   	unsigned int index = vq2txq(sq->vq);
>   	struct netdev_queue *txq;
> +	struct xsk_buff_pool *pool;
> +	int work = 0;
>   
>   	if (unlikely(is_xdp_raw_buffer_queue(vi, index))) {
>   		/* We don't need to enable cb for XDP */
> @@ -1599,15 +1608,26 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
>   
>   	txq = netdev_get_tx_queue(vi->dev, index);
>   	__netif_tx_lock(txq, raw_smp_processor_id());
> -	free_old_xmit_skbs(sq, true);
> +
> +	rcu_read_lock();
> +	pool = rcu_dereference(sq->xsk.pool);
> +	if (pool) {
> +		work = virtnet_xsk_run(sq, pool, budget);
> +		rcu_read_unlock();
> +	} else {
> +		rcu_read_unlock();
> +		free_old_xmit_skbs(sq, true);
> +	}
> +
>   	__netif_tx_unlock(txq);
>   
> -	virtqueue_napi_complete(napi, sq->vq, 0);
> +	if (work < budget)
> +		virtqueue_napi_complete(napi, sq->vq, 0);
>   
>   	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
>   		netif_tx_wake_queue(txq);
>   
> -	return 0;
> +	return work;
>   }
>   
>   static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
> @@ -2647,6 +2667,180 @@ static int virtnet_xdp(struct net_device *dev, struct netdev_bpf *xdp)
>   	}
>   }
>   
> +static int virtnet_xsk_xmit(struct send_queue *sq, struct xsk_buff_pool *pool,
> +			    struct xdp_desc *desc)
> +{
> +	struct virtnet_info *vi = sq->vq->vdev->priv;
> +	void *data, *ptr;
> +	struct page *page;
> +	struct virtnet_xsk_hdr *xskhdr;
> +	u32 idx, offset, n, i, copy, copied;
> +	u64 addr;
> +	int err, m;
> +
> +	addr = desc->addr;
> +
> +	data = xsk_buff_raw_get_data(pool, addr);
> +	offset = offset_in_page(data);
> +
> +	/* one for hdr, one for the first page */
> +	n = 2;
> +	m = desc->len - (PAGE_SIZE - offset);
> +	if (m > 0) {
> +		n += m >> PAGE_SHIFT;
> +		if (m & PAGE_MASK)
> +			++n;
> +
> +		n = min_t(u32, n, ARRAY_SIZE(sq->sg));
> +	}
> +
> +	idx = sq->xsk.hdr_con % sq->xsk.hdr_n;


I don't understand the reason of the hdr array. It looks to me all of 
them are zero and read only from device.

Any reason for not reusing a single hdr for all xdp descriptors? Or 
maybe it's time to introduce VIRTIO_NET_F_NO_HDR.


> +	xskhdr = &sq->xsk.hdr[idx];
> +
> +	/* xskhdr->hdr has been memset to zero, so not need to clear again */
> +
> +	sg_init_table(sq->sg, n);
> +	sg_set_buf(sq->sg, &xskhdr->hdr, vi->hdr_len);
> +
> +	copied = 0;
> +	for (i = 1; i < n; ++i) {
> +		copy = min_t(int, desc->len - copied, PAGE_SIZE - offset);
> +
> +		page = xsk_buff_raw_get_page(pool, addr + copied);
> +
> +		sg_set_page(sq->sg + i, page, copy, offset);
> +		copied += copy;
> +		if (offset)
> +			offset = 0;
> +	}


It looks to me we need to terminate the sg:

**
  * virtqueue_add_outbuf - expose output buffers to other end
  * @vq: the struct virtqueue we're talking about.
  * @sg: scatterlist (must be well-formed and terminated!)


> +
> +	xskhdr->len = desc->len;
> +	ptr = xdp_to_ptr(&xskhdr->type);
> +
> +	err = virtqueue_add_outbuf(sq->vq, sq->sg, n, ptr, GFP_ATOMIC);
> +	if (unlikely(err))
> +		sq->xsk.last_desc = *desc;
> +	else
> +		sq->xsk.hdr_con++;
> +
> +	return err;
> +}
> +
> +static bool virtnet_xsk_dev_is_full(struct send_queue *sq)
> +{
> +	if (sq->vq->num_free < 2 + MAX_SKB_FRAGS)
> +		return true;
> +
> +	if (sq->xsk.hdr_con == sq->xsk.hdr_pro)
> +		return true;


Can we really reach here?


> +
> +	return false;
> +}
> +
> +static int virtnet_xsk_xmit_zc(struct send_queue *sq,
> +			       struct xsk_buff_pool *pool, unsigned int budget)
> +{
> +	struct xdp_desc desc;
> +	int err, packet = 0;
> +	int ret = -EAGAIN;
> +
> +	if (sq->xsk.last_desc.addr) {
> +		err = virtnet_xsk_xmit(sq, pool, &sq->xsk.last_desc);
> +		if (unlikely(err))
> +			return -EBUSY;
> +
> +		++packet;
> +		sq->xsk.last_desc.addr = 0;
> +	}
> +
> +	while (budget-- > 0) {
> +		if (virtnet_xsk_dev_is_full(sq)) {
> +			ret = -EBUSY;
> +			break;
> +		}


It looks to me we will always hit this if userspace is fast. E.g we 
don't kick until the virtqueue is full ...


> +
> +		if (!xsk_tx_peek_desc(pool, &desc)) {
> +			/* done */
> +			ret = 0;
> +			break;
> +		}
> +
> +		err = virtnet_xsk_xmit(sq, pool, &desc);
> +		if (unlikely(err)) {
> +			ret = -EBUSY;
> +			break;
> +		}
> +
> +		++packet;
> +	}
> +
> +	if (packet) {
> +		xsk_tx_release(pool);
> +
> +		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> +			u64_stats_update_begin(&sq->stats.syncp);
> +			sq->stats.kicks++;
> +			u64_stats_update_end(&sq->stats.syncp);
> +		}
> +	}
> +
> +	return ret;
> +}
> +
> +static int virtnet_xsk_run(struct send_queue *sq,
> +			   struct xsk_buff_pool *pool, int budget)
> +{
> +	int err, ret = 0;
> +	unsigned int _packets = 0;
> +	unsigned int _bytes = 0;
> +
> +	sq->xsk.wait_slot = false;
> +
> +	__free_old_xmit_ptr(sq, true, false, &_packets, &_bytes);
> +
> +	err = virtnet_xsk_xmit_zc(sq, pool, xsk_budget);
> +	if (!err) {
> +		struct xdp_desc desc;
> +
> +		clear_bit(VIRTNET_STATE_XSK_WAKEUP, &sq->xsk.state);
> +		xsk_set_tx_need_wakeup(pool);
> +
> +		/* Race breaker. If new is coming after last xmit
> +		 * but before flag change
> +		 */
> +
> +		if (!xsk_tx_peek_desc(pool, &desc))
> +			goto end;
> +
> +		set_bit(VIRTNET_STATE_XSK_WAKEUP, &sq->xsk.state);
> +		xsk_clear_tx_need_wakeup(pool);


How memory ordering is going to work here? Or we don't need to care 
about that?


> +
> +		sq->xsk.last_desc = desc;
> +		ret = budget;
> +		goto end;
> +	}
> +
> +	xsk_clear_tx_need_wakeup(pool);
> +
> +	if (err == -EAGAIN) {
> +		ret = budget;
> +		goto end;
> +	}
> +
> +	__free_old_xmit_ptr(sq, true, false, &_packets, &_bytes);
> +
> +	if (!virtnet_xsk_dev_is_full(sq)) {
> +		ret = budget;
> +		goto end;
> +	}
> +
> +	sq->xsk.wait_slot = true;
> +
> +	virtnet_sq_stop_check(sq, true);
> +end:
> +	return ret;
> +}
> +
>   static int virtnet_get_phys_port_name(struct net_device *dev, char *buf,
>   				      size_t len)
>   {