Re: [PATCH v2 1/3] net/bonding: support Tx prepare

[fengchengwen <fengchengwen@huawei.com>]

Hi Chas,

On 2022/9/15 0:59, Chas Williams wrote:
> On 9/13/22 20:46, fengchengwen wrote:
>>
>> The main problem is hard to design a tx_prepare for bonding device:
>> 1. as Chas Williams said, there maybe twice hash calc to get target slave
>>     devices.
>> 2. also more important, if the slave devices have changes(e.g. slave device
>>     link down or remove), and if the changes happens between bond-tx-prepare and
>>     bond-tx-burst, the output slave will changes, and this may lead to checksum
>>     failed. (Note: a bond device with slave devices may from different vendors,
>>     and slave devices may have different requirements, e.g. slave-A support calc
>>     IPv4 pseudo-head automatic (no need driver pre-calc), but slave-B need driver
>>     pre-calc).
>>
>> Current design cover the above two scenarios by using in-place tx-prepare. and
>> in addition, bond devices are not transparent to applications, I think it's a
>> practical method to provide tx-prepare support in this way.
>>
> 
> 
> I don't think you need to export an enable/disable routine for the use of
> rte_eth_tx_prepare. It's safe to just call that routine, even if it isn't
> implemented. You are just trading one branch in DPDK librte_eth_dev for a
> branch in drivers/net/bonding.

Our first patch was just like yours (just add tx-prepare default), but community
is concerned about impacting performance.

As a trade-off, I think we can add the enable/disable API.

> 
> I think you missed fixing tx_machine in 802.3ad support. We have been using
> the following patch locally which I never got around to submitting.

You are right, I will send V3 fix it.

> 
> 
> From a458654d68ff5144266807ef136ac3dd2adfcd98 Mon Sep 17 00:00:00 2001
> From: "Charles (Chas) Williams" <chwillia@ciena.com>
> Date: Tue, 3 May 2022 16:52:37 -0400
> Subject: [PATCH] net/bonding: call rte_eth_tx_prepare before rte_eth_tx_burst
> 
> Some PMDs might require a call to rte_eth_tx_prepare before sending the
> packets for transmission. Typically, the prepare step handles the VLAN
> headers, but it may need to do other things.
> 
> Signed-off-by: Chas Williams <chwillia@ciena.com>

...

>               * ring if transmission fails so the packet isn't lost.
> @@ -1322,8 +1350,12 @@ bond_ethdev_tx_burst_broadcast(void *queue, struct rte_mbuf **bufs,
>  
>      /* Transmit burst on each active slave */
>      for (i = 0; i < num_of_slaves; i++) {
> -        slave_tx_total[i] = rte_eth_tx_burst(slaves[i], bd_tx_q->queue_id,
> +        uint16_t nb_prep;
> +
> +        nb_prep = rte_eth_tx_prepare(slaves[i], bd_tx_q->queue_id,
>                      bufs, nb_pkts);
> +        slave_tx_total[i] = rte_eth_tx_burst(slaves[i], bd_tx_q->queue_id,
> +                    bufs, nb_prep);

The tx-prepare may edit packet data, and the broadcast mode will send a packet to all slaves,
the packet data is sent and edited at the same time. Is this likely to cause problems ?

>  
>          if (unlikely(slave_tx_total[i] < nb_pkts))
>              tx_failed_flag = 1;