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

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Message-Id: <20230113080016.45505-3-hengqi@linux.alibaba.com>
Date:   Fri, 13 Jan 2023 16:00:08 +0800
From:   Heng Qi <hengqi@...ux.alibaba.com>
To:     netdev@...r.kernel.org, bpf@...r.kernel.org
Cc:     Jason Wang <jasowang@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        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: [PATCH net-next v4 02/10] virtio-net: fix calculation of MTU for single buffer xdp

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'.

Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Acked-by: Jason Wang <jasowang@...hat.com>
---
In fact, this is a fix patch belonging to the net branch, which needs
to be backported to the stable branch.

 drivers/net/virtio_net.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 443aa7b8f0ad..87226dd8d262 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3074,7 +3074,9 @@ static int virtnet_restore_guest_offloads(struct virtnet_info *vi)
 static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 			   struct netlink_ext_ack *extack)
 {
-	unsigned long int max_sz = PAGE_SIZE - sizeof(struct padded_vnet_hdr);
+	unsigned int room = SKB_DATA_ALIGN(VIRTIO_XDP_HEADROOM +
+					   sizeof(struct skb_shared_info));
+	unsigned int max_sz = PAGE_SIZE - room - ETH_HLEN;
 	struct virtnet_info *vi = netdev_priv(dev);
 	struct bpf_prog *old_prog;
 	u16 xdp_qp = 0, curr_qp;
-- 
2.19.1.6.gb485710b