A long time ago I worked on a RHEL5 bug in which kdump hung during boot
on a set of systems.  The systems hung because they never received timer
interrupts during calibrate_delay.  These systems also all had Opteron
processors on a hypertransport bus, bridged to a pci bus via an Nvidia
MCP55 northbridge chip.  After much wrangling I managed to learn from
Nvidia that they have an undocumented register in some versions of that
chip which control how legacy interrupts are send to the cpu complex
when the ioapic isn't active.  Nvidia defaults this register to only
send legacy interrupts to the BSP, so if kdump happens to boot on an AP,
we never get timer interrupts and boom.  I had initially used this quirk
as a workaround, with my intent being to move apic initalization to an
earlier point in the boot process, so the setting of the register would
be irrelevant.  Given the work involved in doing that however, the
fragile nature of the apic initalization code, and the fact that, over
the 2 years since we found this bug, the MCP55 is the only chip which
seems to have this issue, I've figure at this point its likely safer to
just carry the quirk around.  By setting the referenced bits in this
hidden register, interrupts will be broadcast to all cpus when the
ioapic isn't active on the above described systems.
Acked-by: Simon Horman <horms@verge.net.au>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>