netdev - Re: [PATCH net-next 1/5] net: lan966x: Add XDP_PACKET

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Date:   Mon, 14 Nov 2022 15:16:33 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Horatiu Vultur <horatiu.vultur@...rochip.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, ast@...nel.org,
        daniel@...earbox.net, hawk@...nel.org, john.fastabend@...il.com,
        UNGLinuxDriver@...rochip.com
Subject: Re: [PATCH net-next 1/5] net: lan966x: Add XDP_PACKET_HEADROOM

From: Horatiu Vultur <horatiu.vultur@...rochip.com>
Date: Sun, 13 Nov 2022 12:15:55 +0100

> Update the page_pool params to allocate XDP_PACKET_HEADROOM space as
> headroom for all received frames.
> This is needed for when the XDP_TX and XDP_REDIRECT are implemented.
> 
> Signed-off-by: Horatiu Vultur <horatiu.vultur@...rochip.com>
> ---

[...]

> @@ -466,7 +472,8 @@ static struct sk_buff *lan966x_fdma_rx_get_frame(struct lan966x_rx *rx,
>  
>  	skb_mark_for_recycle(skb);
>  
> -	skb_put(skb, FDMA_DCB_STATUS_BLOCKL(db->status));
> +	skb_put(skb, FDMA_DCB_STATUS_BLOCKL(db->status) + XDP_PACKET_HEADROOM);
> +	skb_pull(skb, XDP_PACKET_HEADROOM);

These two must be:

+	skb_reserve(skb, XDP_PACKET_HEADROOM);
 	skb_put(skb, FDMA_DCB_STATUS_BLOCKL(db->status));

i.e. you only need to add reserve, and that's it. It's not only
faster, but also moves ::tail, which is essential.

>  
>  	lan966x_ifh_get_timestamp(skb->data, &timestamp);
>  

[...]

> --- a/drivers/net/ethernet/microchip/lan966x/lan966x_xdp.c
> +++ b/drivers/net/ethernet/microchip/lan966x/lan966x_xdp.c
> @@ -44,8 +44,9 @@ int lan966x_xdp_run(struct lan966x_port *port, struct page *page, u32 data_len)
>  
>  	xdp_init_buff(&xdp, PAGE_SIZE << lan966x->rx.page_order,
>  		      &port->xdp_rxq);
> -	xdp_prepare_buff(&xdp, page_address(page), IFH_LEN_BYTES,
> -			 data_len - IFH_LEN_BYTES, false);
> +	xdp_prepare_buff(&xdp, page_address(page),
> +			 IFH_LEN_BYTES + XDP_PACKET_HEADROOM,
> +			 data_len - IFH_LEN_BYTES - XDP_PACKET_HEADROOM, false);

Are you sure you need to substract %XDP_PACKET_HEADROOM from
@data_len? I think @data_len is the frame length, so headroom is not
included?

>  	act = bpf_prog_run_xdp(xdp_prog, &xdp);
>  	switch (act) {
>  	case XDP_PASS:
> -- 
> 2.38.0

Thanks,
Olek