[xdp-hints] Re: [PATCH net-next v4 1/3] net: stmmac: introduce wrapper for struct xdp_buff

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 13/04/2023 05.25, Song Yoong Siang wrote:
> Introduce struct stmmac_xdp_buff as a preparation to support XDP Rx
> metadata via kfuncs.
> 
> Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
> ---
>   drivers/net/ethernet/stmicro/stmmac/stmmac.h   |  4 ++++
>   .../net/ethernet/stmicro/stmmac/stmmac_main.c  | 18 +++++++++---------
>   2 files changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> index 3d15e1e92e18..ac8ccf851708 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> @@ -92,6 +92,10 @@ struct stmmac_rx_buffer {
>   	dma_addr_t sec_addr;
>   };
>   
> +struct stmmac_xdp_buff {
> +	struct xdp_buff xdp;
> +};
> +
>   struct stmmac_rx_queue {
>   	u32 rx_count_frames;
>   	u32 queue_index;
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index d7fcab057032..6ffce52ca837 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -5188,9 +5188,9 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>   	int status = 0, coe = priv->hw->rx_csum;
>   	unsigned int next_entry = rx_q->cur_rx;
>   	enum dma_data_direction dma_dir;
> +	struct stmmac_xdp_buff ctx = {};

This code trick {} will zero out the struct.

Is this done purpose and really needed?

On x86_64 this unfortunately generates an asm instruction: rep stos

A repeated store string operation, for zeroing out memory, which is
slow. (Because[1] it supports be suspended by an exception or interrupt,
which requires it to store/restore CPU flags).

[1] https://www.felixcloutier.com/x86/rep:repe:repz:repne:repnz#tbl-4-22


>   	unsigned int desc_size;
>   	struct sk_buff *skb = NULL;
> -	struct xdp_buff xdp;
>   	int xdp_status = 0;
>   	int buf_sz;
>   
> @@ -5311,17 +5311,17 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>   			dma_sync_single_for_cpu(priv->device, buf->addr,
>   						buf1_len, dma_dir);
>   
> -			xdp_init_buff(&xdp, buf_sz, &rx_q->xdp_rxq);
> -			xdp_prepare_buff(&xdp, page_address(buf->page),
> +			xdp_init_buff(&ctx.xdp, buf_sz, &rx_q->xdp_rxq);
> +			xdp_prepare_buff(&ctx.xdp, page_address(buf->page),
>   					 buf->page_offset, buf1_len, false);
>   
> -			pre_len = xdp.data_end - xdp.data_hard_start -
> +			pre_len = ctx.xdp.data_end - ctx.xdp.data_hard_start -
>   				  buf->page_offset;
> -			skb = stmmac_xdp_run_prog(priv, &xdp);
> +			skb = stmmac_xdp_run_prog(priv, &ctx.xdp);
>   			/* Due xdp_adjust_tail: DMA sync for_device
>   			 * cover max len CPU touch
>   			 */
> -			sync_len = xdp.data_end - xdp.data_hard_start -
> +			sync_len = ctx.xdp.data_end - ctx.xdp.data_hard_start -
>   				   buf->page_offset;
>   			sync_len = max(sync_len, pre_len);
>   
> @@ -5331,7 +5331,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>   
>   				if (xdp_res & STMMAC_XDP_CONSUMED) {
>   					page_pool_put_page(rx_q->page_pool,
> -							   virt_to_head_page(xdp.data),
> +							   virt_to_head_page(ctx.xdp.data),
>   							   sync_len, true);
>   					buf->page = NULL;
>   					priv->dev->stats.rx_dropped++;
> @@ -5359,7 +5359,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>   
>   		if (!skb) {
>   			/* XDP program may expand or reduce tail */
> -			buf1_len = xdp.data_end - xdp.data;
> +			buf1_len = ctx.xdp.data_end - ctx.xdp.data;
>   
>   			skb = napi_alloc_skb(&ch->rx_napi, buf1_len);
>   			if (!skb) {
> @@ -5369,7 +5369,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>   			}
>   
>   			/* XDP program may adjust header */
> -			skb_copy_to_linear_data(skb, xdp.data, buf1_len);
> +			skb_copy_to_linear_data(skb, ctx.xdp.data, buf1_len);
>   			skb_put(skb, buf1_len);
>   
>   			/* Data payload copied into SKB, page ready for recycle */