netdev - Re: [PATCH net-next] sfc: fix XDP-redirect in this driver

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Date:   Mon, 16 Mar 2020 18:22:58 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     brouer@...hat.com
Cc:     netdev@...r.kernel.org, linux-net-drivers@...arflare.com,
        ecree@...arflare.com, mhabets@...arflare.com,
        cmclachlan@...arflare.com, ilias.apalodimas@...aro.org,
        lorenzo@...nel.org, sameehj@...zon.com
Subject: Re: [PATCH net-next] sfc: fix XDP-redirect in this driver

From: Jesper Dangaard Brouer <brouer@...hat.com>
Date: Fri, 13 Mar 2020 14:25:19 +0100

> XDP-redirect is broken in this driver sfc. XDP_REDIRECT requires
> tailroom for skb_shared_info when creating an SKB based on the
> redirected xdp_frame (both in cpumap and veth).
> 
> The fix requires some initial explaining. The driver uses RX page-split
> when possible. It reserves the top 64 bytes in the RX-page for storing
> dma_addr (struct efx_rx_page_state). It also have the XDP recommended
> headroom of XDP_PACKET_HEADROOM (256 bytes). As it doesn't reserve any
> tailroom, it can still fit two standard MTU (1500) frames into one page.
> 
> The sizeof struct skb_shared_info in 320 bytes. Thus drivers like ixgbe
> and i40e, reduce their XDP headroom to 192 bytes, which allows them to
> fit two frames with max 1536 bytes into a 4K page (192+1536+320=2048).
> 
> The fix is to reduce this drivers headroom to 128 bytes and add the 320
> bytes tailroom. This account for reserved top 64 bytes in the page, and
> still fit two frame in a page for normal MTUs.
> 
> We must never go below 128 bytes of headroom for XDP, as one cacheline
> is for xdp_frame area and next cacheline is reserved for metadata area.
> 
> Fixes: eb9a36be7f3e ("sfc: perform XDP processing on received packets")
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
> Targetted net-next as this is part of a patchset for adding XDP frame
> size and reserving tailroom for multi-buffer XDP

Applied, thanks Jesper.