[Xen-devel] tg3 NIC driver bug in 3.14.x under Xen

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

I am experiencing what appears to be a bug involving the tg3 NIC
driver in (various stable branches of) Linux.

The symptom is a very high level of packet loss: around 25-30% (as
seen in `ping').  There don't seem to be any untoward-looking kernel
messages.  The lost packets get added to the `errors' counter shown in
ifconfig.  I don't know whether the problem is with the transmit path,
or receive path, or both.

All connections and data transfers seem to complete correctly
eventually, but they can be very slow indeed.

The bug occurs only when Linux is running under Xen.  I have
reproduced the bug with Linux running as dom0, both as a 32-bit PAE PV
guest and as a 64-bit PV guest.  I have reproduced the bug with Linux
3.14.21, 3.14.34 and 3.18.0, but the bug seems absent from Debian's
Linux 3.2.0-4-686-pae.


An example of the failure can be seen in the logs from this automated
test:
  
http://logs.test-lab.xenproject.org/osstest/logs/50216/test-amd64-i386-xl/info.html

The host's serial console output is here:
  
http://logs.test-lab.xenproject.org/osstest/logs/50216/test-amd64-i386-xl/serial-elbling0.log
(Up to 01:23:01, at which time the automated tester started log
capture including invoking Xen debug keys.)
  
As you can see from those logs, the test simply times out (in an
operation which involves a lot of data transfer from a cache host on
the local network).

In that particular test, we used:
  Linux             413cb08cebe9fd8107f556eee48b2d40773cacde
  linux-firmware    c530a75c1e6a472b0eb9558310b518f0dfcd8860
  Xen               3a28f760508fb35c430edac17a9efde5aff6d1d5

The host OS is Debian wheezy i386 (32-bit x86).  The kernel was built
for x86 32-bit PAE on Debian wheezy i386.  Xen was built for 64-bit
x86 on Debian wheezy amd64 (64-bit x86).  In each case we used the
default GCC supplied with Debian.

The full information about the kernel build including build
log, kernel config, build outputs, and test harness control variables
etc., are here:
  
http://logs.test-lab.xenproject.org/osstest/logs/50216/build-amd64-pvops/build/
  
http://logs.test-lab.xenproject.org/osstest/logs/50216/build-amd64-pvops/info.html


I have a number of machines which are affected by this bug[1].  All
have a tg3 as onboard NIC.

The bug is easy for me to reproduce.  I'd appreciate opinions on what
this might be and how to go about debugging and fixing it, and more
generally any help or advice.

I can test proposed kernel patches, or debugging patches, easily.
(But if you provide patches please say what they are based on.)

Thanks,
Ian.

[1] For my and Xen community reference:
  - elbling{0,1} in the new osstest test lab
  - merlot{0,1} in the new osstest test lab
  - bedbug, test box under my desk

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