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

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 */

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