Re: [PATCH] ibmveth: set correct gso_size and gso_type

From: Thomas Falcon <redacted>
Date: Thu,  8 Dec 2016 16:40:03 -0600

This patch is based on an earlier one submitted
by Jon Maxwell with the following commit message:

"We recently encountered a bug where a few customers using ibmveth on the
same LPAR hit an issue where a TCP session hung when large receive was
enabled. Closer analysis revealed that the session was stuck because the
one side was advertising a zero window repeatedly.

We narrowed this down to the fact the ibmveth driver did not set gso_size
which is translated by TCP into the MSS later up the stack. The MSS is
used to calculate the TCP window size and as that was abnormally large,
it was calculating a zero window, even although the sockets receive buffer
was completely empty."

We rely on the Virtual I/O Server partition in a pseries
environment to provide the MSS through the TCP header checksum
field. The stipulation is that users should not disable checksum
offloading if rx packet aggregation is enabled through VIOS.

Some firmware offerings provide the MSS in the RX buffer.
This is signalled by a bit in the RX queue descriptor.

Reviewed-by: Brian King <redacted>
Reviewed-by: Pradeep Satyanarayana <redacted>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Jonathan Maxwell <redacted>
Reviewed-by: David Dai <redacted>
Signed-off-by: Thomas Falcon <redacted>

Applied, although mis-using the TCP checksum field for this is kind of
bogus.  I'm surprised there wasn't some other place you could stick
this value, which wouldn't modify the packet contents.