Intel 5000 and 5100 series memory controllers have a known issue if read
completion coalescing is enabled and the PCI-E Maximum Payload Size is
set to 256B.  To work around this issue, disable read completion
coalescing in the memory controller and root complexes.  Unfortunately,
it must always be disabled, even if no 256B MPS devices are present, due
to the possibility of one being hotplugged.

Links to erratas:
http://www.intel.com/content/dam/doc/specification-update/5000-chipset-memory-controller-hub-specification-update.pdf
http://www.intel.com/content/dam/doc/specification-update/5100-memory-controller-hub-chipset-specification-update.pdf

Thanks to Jesse Brandeburg and Ben Hutchings for providing insight into
the problem.
Tested-and-Reported-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NJon Mason <mason@myri.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>