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Re: [Xen-devel] network misbehaviour with gplpv and 2.6.30

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] network misbehaviour with gplpv and 2.6.30
From: Paul Durrant <paul.durrant@xxxxxxxxxx>
Date: Tue, 21 Jul 2009 10:35:34 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Lyon <andrew.lyon@xxxxxxxxx>, Andrew
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James Harper wrote:
With GPLPV under 2.6.30, GPLPV gets the following from the ring:

ring slot n (first buffer):
 status (length) = 54 bytes
 offset = 0
 flags = NETRXF_extra_info (possibly csum too but not relevant)
ring slot n + 1 (extra info)
 gso.size (mss) = 1460

Because NETRXF_extra_info is not set, that's all I get for that packet.

I assume you mean NETRXF_more_data here? Are you saying that ring slot n has only NETRXF_extra_info and *not* NETRXF_more_data?

In the IP header though, the total length is 1544 (which in itself is a
little strange), but obviously I'm not getting a full packet, just the
ETH+IP+TCP header.

According to Andrew Lyon it works fine in previous versions, so this
problem only arises on 2.6.30. I don't know if netfront on Linux suffers
from a similar problem.

I can't identify any changes that could cause this, but if the problem
is in netback either the frags count isn't being set correctly, or
skb->cb (which appears to be used temporarily to hold nr_frags) is
becoming corrupt (set to 0) somehow, but the window where this could
occur is very small and I can't see where it could happen.

Any suggestions as to where to start looking?

(one nice thing is that I have identified a crash that would occur when
the IP header lied about its length!)



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