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

["Song, Yoong Siang" <yoong.siang.song@intel.com>]

On Friday, April 14, 2023 1:10 AM , Jesper Dangaard Brouer <jbrouer@redhat.com> wrote:
>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

Thanks for the useful information and link. I will submit v5 to remove {}.
>
>
>>   	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
>*/