netdev - Re: [PATCH net-next v4 02/10] virtio-net: fix calculation of MTU for single buffer xdp

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Date:   Fri, 13 Jan 2023 22:24:59 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Heng Qi <hengqi@...ux.alibaba.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH net-next v4 02/10] virtio-net: fix calculation of MTU
 for single buffer xdp

On Fri, 13 Jan 2023 16:00:08 +0800 Heng Qi wrote:
> When single-buffer xdp is loaded, the size of the buffer filled each time
> is 'sz = (PAGE_SIZE - headroom - tailroom)', which is the maximum packet
> length that the driver allows the device to pass in. Otherwise, the packet
> with a length greater than sz will come in, so num_buf will be greater than
> or equal to 2, and then xdp_linearize_page() will be performed and the
> packet will be dropped because the total length is greater than PAGE_SIZE.
> So the maximum value of MTU for single-buffer xdp is 'max_sz = sz - ETH_HLEN'.

drivers/net/virtio_net.c:3111:56: warning: format specifies type 'unsigned long' but the argument has type 'unsigned int' [-Wformat]
                netdev_warn(dev, "XDP requires MTU less than %lu\n", max_sz);
                                                             ~~~     ^~~~~~
                                                             %u