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    SubjectRe: [PATCH net] ixgbe: allow to increase MTU to some extent with XDP enalbed
    On Sat, Jan 21, 2023 at 04:55:21PM +0800, Jason Xing wrote:
    > From: Jason Xing <kernelxing@tencent.com>
    >
    > 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@tencent.com>
    > ---
    > 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);

    This will result with unnecessary overhead for 1500 MTU because you will
    be working on order-1 pages. Instead I would focus on fixing
    ixgbe_change_mtu() and stop relying on ixgbe_rx_bufsz() in there. You can
    check what we do on ice/i40e sides.

    I'm not looking actively into ixgbe internals but I don't think that there
    is anything that stops us from using 3k buffers with XDP.

    > #endif
    > }
    > }
    > --
    > 2.37.3
    >

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