[RFC] ethdev: TX mbuf fast release optimization

[Konstantin Ananyev]

> For TX mbuf fast release offload, I propose to add the mbuf mempool
> pointer to the ethdev tx queue configuration structure,
> so the ethdev TX burst operation doesn't need to fetch it from the
> first mbuf of each burst being fast free'd to the mempool.
> 
> This modification of the struct rte_eth_txconf, and the requirement
> to set the mempool pointer if the RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE
> flag is set, will be an API+ABI change in 25.11.
> Should it be announced in the 25.07 release notes?
> 
> Note: We could phase it in softly by letting the ethdev drivers
> check if the pointer has been set, and fall back to fetching it
> from mbuf[0] if not.
> 
> /**
>  * A structure used to configure a Tx ring of an Ethernet port.
>  */
> struct rte_eth_txconf {
> 	struct rte_eth_thresh tx_thresh; /**< Tx ring threshold registers. */
> 	uint16_t tx_rs_thresh; /**< Drives the setting of RS bit on TXDs. */
> 	uint16_t tx_free_thresh; /**< Start freeing Tx buffers if there are
> 				      less free descriptors than this value. */
> 
> 	uint8_t tx_deferred_start; /**< Do not start queue with rte_eth_dev_start(). */
> 	/**
> 	 * Per-queue Tx offloads to be set  using RTE_ETH_TX_OFFLOAD_* flags.
> 	 * Only offloads set on tx_queue_offload_capa or tx_offload_capa
> 	 * fields on rte_eth_dev_info structure are allowed to be set.
> 	 */
> 	uint64_t offloads;
> 
> +	/**
> +	 * Per-queue mempool to release the mbufs to; required for
> +	 * RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE offload.
> +	 */
> +	struct rte_mempool *mp;
> +

Even though I usually recommend to use MBUF_FAST_FREE -
that's probably a good change.
At least people will realize that they have to provide a single mempool
per TX queue when they enable FAST_FREE flag.
One naming suggestion I have - can we name it somehow more informative:
'fast_free_mp' or so?
Also, we can update tx_queue_setup() to catch the situation when FAST_FREE
is set but mp is NULL, or visa-versa.
Again drivers can probably add extra check when debug is enabled, that
all mbufs are exactly from that mempool.   
 

> 	uint64_t reserved_64s[2]; /**< Reserved for future fields */
> 	void *reserved_ptrs[2];   /**< Reserved for future fields */
> };
>