Re: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support

[Paolo Abeni <pabeni@redhat.com>]

On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote:
> @@ -2661,6 +2776,54 @@ static int tun_ptr_peek_len(void *ptr)
>  	}
>  }
>  
> +static void tun_peek_xsk(struct tun_file *tfile)
> +{
> +	struct xsk_buff_pool *pool;
> +	u32 i, batch, budget;
> +	void *frame;
> +
> +	if (!ptr_ring_empty(&tfile->tx_ring))
> +		return;
> +
> +	spin_lock(&tfile->pool_lock);
> +	pool = tfile->xsk_pool;
> +	if (!pool) {
> +		spin_unlock(&tfile->pool_lock);
> +		return;
> +	}
> +
> +	if (tfile->nb_descs) {
> +		xsk_tx_completed(pool, tfile->nb_descs);
> +		if (xsk_uses_need_wakeup(pool))
> +			xsk_set_tx_need_wakeup(pool);
> +	}
> +
> +	spin_lock(&tfile->tx_ring.producer_lock);
> +	budget = min_t(u32, tfile->tx_ring.size, TUN_XDP_BATCH);
> +
> +	batch = xsk_tx_peek_release_desc_batch(pool, budget);
> +	if (!batch) {

This branch looks like an unneeded "optimization". The generic loop
below should have the same effect with no measurable perf delta - and
smaller code. Just remove this.

> +		tfile->nb_descs = 0;
> +		spin_unlock(&tfile->tx_ring.producer_lock);
> +		spin_unlock(&tfile->pool_lock);
> +		return;
> +	}
> +
> +	tfile->nb_descs = batch;
> +	for (i = 0; i < batch; i++) {
> +		/* Encode the XDP DESC flag into lowest bit for consumer to differ
> +		 * XDP desc from XDP buffer and sk_buff.
> +		 */
> +		frame = tun_xdp_desc_to_ptr(&pool->tx_descs[i]);
> +		/* The budget must be less than or equal to tx_ring.size,
> +		 * so enqueuing will not fail.
> +		 */
> +		__ptr_ring_produce(&tfile->tx_ring, frame);
> +	}
> +	spin_unlock(&tfile->tx_ring.producer_lock);
> +	spin_unlock(&tfile->pool_lock);

More related to the general design: it looks wrong. What if
get_rx_bufs() will fail (ENOBUF) after successful peeking? With no more
incoming packets, later peek will return 0 and it looks like that the
half-processed packets will stay in the ring forever???

I think the 'ring produce' part should be moved into tun_do_read().

Cheers,

Paolo