netdev - Re: [Intel-wired-lan] [PATCH v4 net-next 6/9] igc: don't reserve excessive XDP_PACKET

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Date:   Mon, 27 Dec 2021 22:34:10 +0200
From:   "Kraus, NechamaX" <nechamax.kraus@...ux.intel.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        intel-wired-lan@...ts.osuosl.org
Cc:     Song Liu <songliubraving@...com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        KP Singh <kpsingh@...nel.org>, bpf@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [Intel-wired-lan] [PATCH v4 net-next 6/9] igc: don't reserve
 excessive XDP_PACKET_HEADROOM on XSK Rx to skb

On 12/8/2021 16:06, Alexander Lobakin wrote:
> {__,}napi_alloc_skb() allocates and reserves additional NET_SKB_PAD
> + NET_IP_ALIGN for any skb.
> OTOH, igc_construct_skb_zc() currently allocates and reserves
> additional `xdp->data_meta - xdp->data_hard_start`, which is about
> XDP_PACKET_HEADROOM for XSK frames.
> There's no need for that at all as the frame is post-XDP and will
> go only to the networking stack core.
> Pass the size of the actual data only (+ meta) to
> __napi_alloc_skb() and don't reserve anything. This will give
> enough headroom for stack processing.
> Also, net_prefetch() xdp->data_meta and align the copy size to
> speed-up memcpy() a little and better match igc_costruct_skb().
> 
> Fixes: fc9df2a0b520 ("igc: Enable RX via AF_XDP zero-copy")
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>
> Reviewed-by: Michal Swiatkowski <michal.swiatkowski@...ux.intel.com>
> ---
>   drivers/net/ethernet/intel/igc/igc_main.c | 13 +++++++------
>   1 file changed, 7 insertions(+), 6 deletions(-)
>
Tested-by: Nechama Kraus <nechamax.kraus@...ux.intel.com>