[PATCH v3 2/3] net/i40e: enable direct rearm with separate API

[Konstantin Ananyev]

> > -----邮件原件-----
> > 发件人: Konstantin Ananyev <konstantin.ananyev at huawei.com>
> > 发送时间: Tuesday, February 28, 2023 3:36 AM
> > 收件人: Feifei Wang <Feifei.Wang2 at arm.com>; Konstantin Ananyev
> > <konstantin.v.ananyev at yandex.ru>; Yuying Zhang
> > <Yuying.Zhang at intel.com>; Beilei Xing <beilei.xing at intel.com>; Ruifeng
> > Wang <Ruifeng.Wang at arm.com>
> > 抄送: dev at dpdk.org; nd <nd at arm.com>; Honnappa Nagarahalli
> > <Honnappa.Nagarahalli at arm.com>; nd <nd at arm.com>
> > 主题: RE: [PATCH v3 2/3] net/i40e: enable direct rearm with separate API
> >
> >
> >
> > > > > +int
> > > > > +i40e_tx_fill_sw_ring(void *tx_queue,
> > > > > +		struct rte_eth_rxq_rearm_data *rxq_rearm_data) {
> > > > > +	struct i40e_tx_queue *txq = tx_queue;
> > > > > +	struct i40e_tx_entry *txep;
> > > > > +	void **rxep;
> > > > > +	struct rte_mbuf *m;
> > > > > +	int i, n;
> > > > > +	int nb_rearm = 0;
> > > > > +
> > > > > +	if (*rxq_rearm_data->rearm_nb < txq->tx_rs_thresh ||
> > > > > +			txq->nb_tx_free > txq->tx_free_thresh)
> > > > > +		return 0;
> > > > > +
> > > > > +	/* check DD bits on threshold descriptor */
> > > > > +	if ((txq->tx_ring[txq->tx_next_dd].cmd_type_offset_bsz &
> > > > > +			rte_cpu_to_le_64(I40E_TXD_QW1_DTYPE_MASK)) !=
> > > > > +
> > > > 	rte_cpu_to_le_64(I40E_TX_DESC_DTYPE_DESC_DONE))
> > > > > +		return 0;
> > > > > +
> > > > > +	n = txq->tx_rs_thresh;
> > > > > +
> > > > > +	/* first buffer to free from S/W ring is at index
> > > > > +	 * tx_next_dd - (tx_rs_thresh-1)
> > > > > +	 */
> > > > > +	txep = &txq->sw_ring[txq->tx_next_dd - (n - 1)];
> > > > > +	rxep = rxq_rearm_data->rx_sw_ring;
> > > > > +	rxep += *rxq_rearm_data->rearm_start;
> > > > > +
> > > > > +	if (txq->offloads & RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE) {
> > > > > +		/* directly put mbufs from Tx to Rx */
> > > > > +		for (i = 0; i < n; i++, rxep++, txep++)
> > > > > +			*rxep = txep[0].mbuf;
> > > > > +	} else {
> > > > > +		for (i = 0; i < n; i++, rxep++) {
> > > > > +			m = rte_pktmbuf_prefree_seg(txep[i].mbuf);
> >
> > One thing I forgot to ask:
> > What would happen if this mbuf belongs to different mempool (not one that
> > we specify at rx_queue_setup())?
> > Do we need to check it here?
> > Or would it be upper layer constraint?
> > Or...?
> >
> 
> First, 'different mempool' is valid for no FAST_FREE path in tx_free_buffers.
> 
> If buffers belong to different mempool, we can have an example here:
> Buffer 1 from mempool 1, its recycle path is:
> -----------------------------------------------------------------------------------------
> 1. queue_setup: rearm from mempool 1 into Rx sw-ring
> 2. rte_eth_Rx_burst: used by user app (Rx)
> 3. rte_eth_Tx_burst: mount on Tx sw-ring
> 4. rte_eth_direct_rearm: free into Rx sw-ring:
>            or
>     tx_free_buffers: free into mempool 1 (no fast_free path)
> -----------------------------------------------------------------------------------------
> 
> Buffer 2 from mempool 2, its recycle path is:
> -----------------------------------------------------------------------------------------
> 1. queue_setup: rearm from mempool 2 into Rx sw-ring
> 2. rte_eth_Rx_burst: used by user app (Rx)
> 3. rte_eth_Tx_burst: mount on Tx sw-ring
> 4. rte_eth_direct_rearm: free into Rx sw-ring
>            or
>     tx_free_buffers: free into mempool 2 (no fast_free_path)
> -----------------------------------------------------------------------------------------
> 
> Thus, buffers from Tx different mempools are the same for Rx. The difference point
> is that they will be freed into different mempool if the thread  uses generic free buffers.
> I think this cannot affect direct-rearm mode, and we do not need to check this.

I understand that it should work even with multiple mempools.
What I am trying to say - user may not want to use mbufs from particular mempool for RX
(while it is still ok to use it for TX).
Let say user can have a separate mempool with small data-buffers (less then normal MTU)  
to send some 'special' paclets, or even use this memppol with small buffers for zero-copy
updating of packet L2/L3 headers, etc.
Or it could be some 'special' user provided mempool.
That's why I wonder should we allow only mbufs from mempool that is assigned to that RX queue. 

> 
> > > > > +			if (m != NULL) {
> > > > > +				*rxep = m;
> > > > > +				nb_rearm++;
> > > > > +			}
> > > > > +		}
> > > > > +		n = nb_rearm;
> > > > > +	}
> > > > > +
> > > > > +	/* update counters for Tx */
> > > > > +	txq->nb_tx_free = (uint16_t)(txq->nb_tx_free + txq->tx_rs_thresh);
> > > > > +	txq->tx_next_dd = (uint16_t)(txq->tx_next_dd + txq->tx_rs_thresh);
> > > > > +	if (txq->tx_next_dd >= txq->nb_tx_desc)
> > > > > +		txq->tx_next_dd = (uint16_t)(txq->tx_rs_thresh - 1);
> > > > > +
> > > > > +	return n;
> > > > > +}
> > > > > +