netdev - Re: [RFT] net: ethernet: enetc: do not always access skb_shared

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Date:   Tue, 13 Dec 2022 21:55:51 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, claudiu.manoil@....com,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, lorenzo.bianconi@...hat.com
Subject: Re: [RFT] net: ethernet: enetc: do not always access skb_shared_info
 in the XDP path

Hi Lorenzo,

On Tue, Dec 13, 2022 at 10:46:43AM +0100, Lorenzo Bianconi wrote:
> Move XDP skb_shared_info structure initialization in from
> enetc_map_rx_buff_to_xdp() to enetc_add_rx_buff_to_xdp() and do not always
> access skb_shared_info in the xdp_buff/xdp_frame since it is located in a
> different cacheline with respect to hard_start and data xdp pointers.
> Rely on XDP_FLAGS_HAS_FRAGS flag to check if it really necessary to access
> non-linear part of the xdp_buff/xdp_frame.
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
> This patch is based on the following series not applied yet to next-next:
> https://patchwork.kernel.org/project/netdevbpf/cover/cover.1670680119.git.lorenzo@kernel.org/
> ---
>  drivers/net/ethernet/freescale/enetc/enetc.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
> index cd8f5f0c6b54..2ed6b163f3c8 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc.c
> +++ b/drivers/net/ethernet/freescale/enetc/enetc.c
> @@ -1305,6 +1305,10 @@ static int enetc_xdp_frame_to_xdp_tx_swbd(struct enetc_bdr *tx_ring,
>  	xdp_tx_swbd->xdp_frame = NULL;
>  
>  	n++;
> +
> +	if (!xdp_frame_has_frags(xdp_frame))
> +		goto out;
> +

Tested this with single-buffer devmap XDP_REDIRECT, can't test with
multi-buffer I think.

>  	xdp_tx_swbd = &xdp_tx_arr[n];
>  
>  	shinfo = xdp_get_shared_info_from_frame(xdp_frame);
> @@ -1334,7 +1338,7 @@ static int enetc_xdp_frame_to_xdp_tx_swbd(struct enetc_bdr *tx_ring,
>  		n++;
>  		xdp_tx_swbd = &xdp_tx_arr[n];
>  	}
> -
> +out:
>  	xdp_tx_arr[n - 1].is_eof = true;
>  	xdp_tx_arr[n - 1].xdp_frame = xdp_frame;
>  
> @@ -1390,16 +1394,12 @@ static void enetc_map_rx_buff_to_xdp(struct enetc_bdr *rx_ring, int i,
>  {
>  	struct enetc_rx_swbd *rx_swbd = enetc_get_rx_buff(rx_ring, i, size);
>  	void *hard_start = page_address(rx_swbd->page) + rx_swbd->page_offset;
> -	struct skb_shared_info *shinfo;
>  
>  	/* To be used for XDP_TX */
>  	rx_swbd->len = size;
>  
>  	xdp_prepare_buff(xdp_buff, hard_start - rx_ring->buffer_offset,
>  			 rx_ring->buffer_offset, size, false);
> -
> -	shinfo = xdp_get_shared_info_from_buff(xdp_buff);
> -	shinfo->nr_frags = 0;
>  }
>  
>  static void enetc_add_rx_buff_to_xdp(struct enetc_bdr *rx_ring, int i,
> @@ -1407,7 +1407,7 @@ static void enetc_add_rx_buff_to_xdp(struct enetc_bdr *rx_ring, int i,
>  {
>  	struct skb_shared_info *shinfo = xdp_get_shared_info_from_buff(xdp_buff);
>  	struct enetc_rx_swbd *rx_swbd = enetc_get_rx_buff(rx_ring, i, size);
> -	skb_frag_t *frag = &shinfo->frags[shinfo->nr_frags];
> +	skb_frag_t *frag;
>  
>  	/* To be used for XDP_TX */
>  	rx_swbd->len = size;
> @@ -1415,6 +1415,7 @@ static void enetc_add_rx_buff_to_xdp(struct enetc_bdr *rx_ring, int i,
>  	if (!xdp_buff_has_frags(xdp_buff)) {
>  		xdp_buff_set_frags_flag(xdp_buff);
>  		shinfo->xdp_frags_size = size;
> +		shinfo->nr_frags = 0;

Tested this and enetc_map_rx_buff_to_xdp() with single-buffer and
multi-buffer XDP_TX.

>  	} else {
>  		shinfo->xdp_frags_size += size;
>  	}
> @@ -1422,6 +1423,7 @@ static void enetc_add_rx_buff_to_xdp(struct enetc_bdr *rx_ring, int i,
>  	if (page_is_pfmemalloc(rx_swbd->page))
>  		xdp_buff_set_frag_pfmemalloc(xdp_buff);
>  
> +	frag = &shinfo->frags[shinfo->nr_frags];
>  	skb_frag_off_set(frag, rx_swbd->page_offset);
>  	skb_frag_size_set(frag, size);
>  	__skb_frag_set_page(frag, rx_swbd->page);
> -- 
> 2.38.1
>

Reviewed-by: Vladimir Oltean <vladimir.oltean@....com>
Tested-by: Vladimir Oltean <vladimir.oltean@....com>

Thanks.