netdev - Re: [PATCH] ibmveth: set correct gso_size and gso

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Date:   Fri, 09 Dec 2016 22:48:15 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     tlfalcon@...ux.vnet.ibm.com
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH] ibmveth: set correct gso_size and gso_type

From: Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>
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 <brking@...ux.vnet.ibm.com>
> Reviewed-by: Pradeep Satyanarayana <pradeeps@...ux.vnet.ibm.com>
> Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> Reviewed-by: Jonathan Maxwell <jmaxwell37@...il.com>
> Reviewed-by: David Dai <zdai@...ibm.com>
> Signed-off-by: Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>

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.