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From:   Jason Xing <kerneljasonxing@gmail.com>
To:     jesse.brandeburg@intel.com, anthony.l.nguyen@intel.com,
        davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
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Cc:     intel-wired-lan@lists.osuosl.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        kerneljasonxing@gmail.com, Jason Xing <kernelxing@tencent.com>
Subject: [PATCH net] ixgbe: allow to increase MTU to some extent with XDP enalbed
Date:   Sat, 21 Jan 2023 16:55:21 +0800
Message-Id: <20230121085521.9566-1-kerneljasonxing@gmail.com>
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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);
 #endif
 	}
 }
-- 
2.37.3