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

Commit Message

Heng Qi Jan. 13, 2023, 8 a.m. UTC

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 <[e-mail]>
Reviewed-by: Xuan Zhuo <[e-mail]>
Acked-by: Jason Wang <[e-mail]>
---
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(-)

Comments

Jakub Kicinski Jan. 14, 2023, 6:24 a.m. UTC | #1

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

Heng Qi Jan. 14, 2023, 7:16 a.m. UTC | #2

在 2023/1/14 下午2:24, Jakub Kicinski 写道:
> 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

The corresponding netdev_warn are split into [PATCH net-next v4 03/10], 
and I will reorganize the patches [PATCH net-next v4 02/10] and [PATCH 
net-next v4 03/10] to be more independent.

Thanks.

Patch

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;