在 2022/12/28 11:50, Heng Qi 写道:

在 2022/12/27 下午8:20, Heng Qi 写道:

quoted

在 2022/12/27 下午2:32, Jason Wang 写道:

quoted

在 2022/12/20 22:14, Heng Qi 写道:

quoted

When the xdp program sets xdp.frags, which means it can process
multi-buffer packets over larger MTU, so we continue to support xdp.
But for single-buffer xdp, we should keep checking for MTU.

Signed-off-by: Heng Qi <redacted>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
  drivers/net/virtio_net.c | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 443aa7b8f0ad..c5c4e9db4ed3 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -3095,8 +3095,8 @@ static int virtnet_xdp_set(struct net_device 

*dev, struct bpf_prog *prog,
          return -EINVAL;
      }
  -    if (dev->mtu > max_sz) {
-        NL_SET_ERR_MSG_MOD(extack, "MTU too large to enable XDP");
+    if (prog && !prog->aux->xdp_has_frags && dev->mtu > max_sz) {

Not related to this patch, but I see:

        unsigned long int max_sz = PAGE_SIZE - sizeof(struct 
padded_vnet_hdr);

Which is suspicious, do we need to count reserved headroom/tailroom 
as well?

This seems to be suspicious. After loading xdp, the size of the 
filled avail buffer
is (PAGE_SIZE - headroom - tailroom), so the size of the received 
used buffer, ie MTU,
should also be (PAGE_SIZE - headroom - tailroom).

Hi Jason, this is indeed a problem. After verification, packet drop 
will indeed occur.  To avoid this,
the size of MTU should be (PAGE_SIZE - headroom - tailroom - ethhdr = 
4096 - 256 -320 - 14 =3506).
Because when there is xdp, each filling is 3520 (PAGE_SIZE - room), if 
the value of (MTU + 14) is
greater than 3520 (because the MTU does not contain the ethernet 
header), then the packet with a
length greater than 3520 will come in, so num_buf will still 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.

I will make a separate bugfix patch to fix this later.

Great.

Thanks

Thanks.

quoted

Thanks.

quoted

Thanks

quoted

+        NL_SET_ERR_MSG_MOD(extack, "MTU too large to enable XDP 
without frags");
          netdev_warn(dev, "XDP requires MTU less than %lu\n", 
max_sz);
          return -EINVAL;
      }