[RFC 4/4] net/ice: remove use of VLAs

[Ferruh Yigit]

On 5/23/2024 5:26 PM, Konstantin Ananyev wrote:
> From: Konstantin Ananyev <konstantin.ananyev at huawei.com>
> 
> ../drivers/net/ice/ice_rxtx.c:1871:29: warning: variable length array used [-Wvla]
> 
> Here VLA is used as a temp array for mbufs that will be used as a split
> RX data buffers.
> As at any given time only one thread can do RX from particular queue,
> at rx_queue_setup() we can allocate extra space for that array, and then
> safely use it at RX fast-path.
> 

Is there a reason to allocate extra space in sw_ring and used some part
of it for split buffer, instead of allocating a new buffer for it?

> Signed-off-by: Konstantin Ananyev <konstantin.ananyev at huawei.com>
> ---
>  drivers/net/ice/ice_rxtx.c | 18 ++++++++++++------
>  drivers/net/ice/ice_rxtx.h |  2 ++
>  2 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ice/ice_rxtx.c b/drivers/net/ice/ice_rxtx.c
> index 95a2db3432..6395a3b50a 100644
> --- a/drivers/net/ice/ice_rxtx.c
> +++ b/drivers/net/ice/ice_rxtx.c
> @@ -1171,7 +1171,7 @@ ice_rx_queue_setup(struct rte_eth_dev *dev,
>  	struct ice_vsi *vsi = pf->main_vsi;
>  	struct ice_rx_queue *rxq;
>  	const struct rte_memzone *rz;
> -	uint32_t ring_size;
> +	uint32_t ring_size, tlen;
>  	uint16_t len;
>  	int use_def_burst_func = 1;
>  	uint64_t offloads;
> @@ -1279,9 +1279,14 @@ ice_rx_queue_setup(struct rte_eth_dev *dev,
>  	/* always reserve more for bulk alloc */
>  	len = (uint16_t)(nb_desc + ICE_RX_MAX_BURST);
>  
> +	/* allocate extra entries for SW split buffer */
> +	tlen = ((rxq->offloads & RTE_ETH_RX_OFFLOAD_BUFFER_SPLIT) != 0) ?
> +		rxq->rx_free_thresh : 0;
> +	tlen += len;
> +
>  	/* Allocate the software ring. */
>  	rxq->sw_ring = rte_zmalloc_socket(NULL,
> -					  sizeof(struct ice_rx_entry) * len,
> +					  sizeof(struct ice_rx_entry) * tlen,
>  					  RTE_CACHE_LINE_SIZE,
>  					  socket_id);
>  	if (!rxq->sw_ring) {
> @@ -1290,6 +1295,8 @@ ice_rx_queue_setup(struct rte_eth_dev *dev,
>  		return -ENOMEM;
>  	}
>  
> +	rxq->sw_split_buf = (tlen == len) ? NULL : rxq->sw_ring + len;
> +
>  	ice_reset_rx_queue(rxq);
>  	rxq->q_set = true;
>  	dev->data->rx_queues[queue_idx] = rxq;
> @@ -1868,7 +1875,6 @@ ice_rx_alloc_bufs(struct ice_rx_queue *rxq)
>  	uint64_t dma_addr;
>  	int diag, diag_pay;
>  	uint64_t pay_addr;
> -	struct rte_mbuf *mbufs_pay[rxq->rx_free_thresh];
>  
>  	/* Allocate buffers in bulk */
>  	alloc_idx = (uint16_t)(rxq->rx_free_trigger -
> @@ -1883,7 +1889,7 @@ ice_rx_alloc_bufs(struct ice_rx_queue *rxq)
>  
>  	if (rxq->offloads & RTE_ETH_RX_OFFLOAD_BUFFER_SPLIT) {
>  		diag_pay = rte_mempool_get_bulk(rxq->rxseg[1].mp,
> -				(void *)mbufs_pay, rxq->rx_free_thresh);
> +				(void *)rxq->sw_split_buf, rxq->rx_free_thresh);
>

Are we allowed to call 'rte_mempool_get_bulk()' with NULL object_table,
as 'rxq->sw_split_buf' can be NULL?
Perhaps can allocate 'rxq->sw_split_buf' even buffer split offload is
not enabled?