[Intel-wired-lan] i40e XDP - inform user if MTU exceeds rx_buf_len when loading program

[Garrett K]

Hi,

A coworker reached out to me after updating distributions on a lab machine.
A previously-working eBFP/XDP program suddenly stopped working, reporting
"invalid argument" when attempting to load it with iproute2.

write(2, "RTNETLINK answers: Invalid argum"..., 36RTNETLINK answers:
Invalid argument

It looks like their MTU was somehow set to 9000 after the update. Reducing
the MTU allowed the program to load.

I _think_ that error was coming from i40e_main.c:i40e_xdp_setup(). If
that's the indeed true, and if it's possible, it might be helpful to bubble
an error message back up to the user. Maybe something like this?

diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c
b/drivers/net/ethernet/intel/i40e/i40e_main.c
index f52e2c46e6a7..134ecbbbb84f 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_main.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
@@ -11872,8 +11872,11 @@ static int i40e_xdp_setup(struct i40e_vsi *vsi,
        int i;

        /* Don't allow frames that span over multiple buffers */
-       if (frame_size > vsi->rx_buf_len)
+       if (frame_size > vsi->rx_buf_len) {
+               netdev_warn(vsi->netdev, "Frame size %d exceeds rx_buf_len
%d; reduce device MTU.\n",
+                       frame_size, vsi->rx_buf_len);
                return -EINVAL;
+       }

Thank you for the consideration, and apologies if this isn't the right list
for this report.

Cheers,

Garrett Kuchta
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