Re: eth_get_headlen() and unaligned accesses...

On Thu, Oct 9, 2014 at 5:12 PM, David Miller [off-list ref] wrote:

So, we have a bit of a problem, this is on sparc64:

[423475.740836] Kernel unaligned access at TPC[81d330] __skb_flow_get_ports+0x70/0xe0
[423475.755756] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.17.0+ #2
[423475.767854] Call Trace:
[423475.772877]  [0000000000433288] kernel_unaligned_trap+0x368/0x5c0
[423475.785203]  [000000000042a824] sun4v_do_mna+0x84/0xa0
[423475.795624]  [0000000000406cd0] sun4v_mna+0x5c/0x68
[423475.805521]  [000000000081d330] __skb_flow_get_ports+0x70/0xe0
[423475.817323]  [000000000081d6ac] __skb_flow_dissect+0x1ac/0x460
[423475.829128]  [0000000000843c98] eth_get_headlen+0x38/0xa0
[423475.840083]  [0000000010064d54] igb_poll+0x8d4/0xf60 [igb]
[423475.851184]  [00000000008243c8] net_rx_action+0xa8/0x1c0

The chip DMA's to the beginning of a frag page and (unless timestamps
are enabled) that's where the ethernet header begins.

So any larger than 16-bit access to the IP and later headers will be
unaligned.

Will this also be a problem for GRE packet carrying Ethernet packet?

We have various ways we can deal with this based upon the capabilities
of the chips involved.  Can we configure the IGB to put 2 "don't care"
bytes at the beginning of the packet?
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