Re: [PATCH net] ixgbe: allow to increase MTU to some extent with XDP enalbed

[Jason Xing]

I'm sorry. I just noticed that I sent it to the wrong email address of
john.fastabend previously. So I corrected it here.

Thanks,
Jason

On Sat, Jan 21, 2023 at 4:55 PM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
>
> From: Jason Xing <kernelxing@xxxxxxxxxxx>
>
> I encountered one case where I cannot increase the MTU size with XDP
> enabled if the server is equipped with IXGBE card, which happened on
> thousands of servers. I noticed it was prohibited from 2017[1] and
> added size checks[2] if allowed soon after the previous patch.
>
> Interesting part goes like this:
> 1) Changing MTU directly from 1500 (default value) to 2000 doesn't
> work because the driver finds out that 'new_frame_size >
> ixgbe_rx_bufsz(ring)' in ixgbe_change_mtu() function.
> 2) However, if we change MTU to 1501 then change from 1501 to 2000, it
> does work, because the driver sets __IXGBE_RX_3K_BUFFER when MTU size
> is converted to 1501, which later size check policy allows.
>
> The default MTU value for most servers is 1500 which cannot be adjusted
> directly to the value larger than IXGBE_MAX_2K_FRAME_BUILD_SKB (1534 or
> 1536) if it loads XDP.
>
> After I do a quick study on the manner of i40E driver allowing two kinds
> of buffer size (one is 2048 while another is 3072) to support XDP mode in
> i40e_max_xdp_frame_size(), I believe the default MTU size is possibly not
> satisfied in XDP mode when IXGBE driver is in use, we sometimes need to
> insert a new header, say, vxlan header. So setting the 3K-buffer flag
> could solve the issue.
>
> [1] commit 38b7e7f8ae82 ("ixgbe: Do not allow LRO or MTU change with XDP")
> [2] commit fabf1bce103a ("ixgbe: Prevent unsupported configurations with
> XDP")
>
> Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx>
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index ab8370c413f3..dc016582f91e 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -4313,6 +4313,9 @@ static void ixgbe_set_rx_buffer_len(struct ixgbe_adapter *adapter)
>                 if (IXGBE_2K_TOO_SMALL_WITH_PADDING ||
>                     (max_frame > (ETH_FRAME_LEN + ETH_FCS_LEN)))
>                         set_bit(__IXGBE_RX_3K_BUFFER, &rx_ring->state);
> +
> +               if (ixgbe_enabled_xdp_adapter(adapter))
> +                       set_bit(__IXGBE_RX_3K_BUFFER, &rx_ring->state);
>  #endif
>         }
>  }
> --
> 2.37.3
>