[Xen-devel] question on large send offload

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

Does the data for a large send offload have to occupy more than one
page? In tuning the windows drivers, if I set the minimum mss multiple
before considering offload (a windows variable) to 2, then I seem to get
hangs. If I set it to 4 then everything appears to be okay. 

If the mss is 1460 (normal for Ethernet), then 2 * 1460 = 2920, which
will often fit into one page, in which case the ring looks like:

0: tx_request - flags |= NETTXF_extra_info
1: extra_info

If the 2920 bytes were on more than one page, then the ring would look
like:

0: tx_request - flags |= NETTXF_extra_info | NETTXF_more_data
1: extra_info
2: tx_request - no change to flags

The behaviour I'm seeing is that I'll get a bunch of packets working
just fine, then it will pause for a bit, and I'm wondering if I could be
getting a bunch of the latter above case, and then one of the former
which causes the hang. I haven't yet put debug statements in to find out
which of the above is the packet before the hang... I'll do that
tomorrow.

Any suggestions?

Thanks

James

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