On Intel systems corrected machine check interrupts (CMCI) may be sent to
multiple logical processors; possibly to all processors on the affected
socket (SDM Volume 3B "15.5.1 CMCI Local APIC Interface").  This means
that a persistent error (such as a stuck bit in ECC memory) may cause
a storm of interrupts that greatly hinders or prevents forward progress
(probably on many processors).

To solve this we keep track of the rate at which each processor sees
CMCI. If we exceed a threshold, we disable CMCI delivery and switch to
polling the machine check banks. If the storm subsides (none of the
affected processors see any more errors for a complete poll interval) we
re-enable CMCI.

[Tony: Added console messages when storm begins/ends and increased storm
threshold from 5 to 15 so we have a few more logged entries before we
disable interrupts and start dropping reports]

Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Chen Gong <gong.chen@linux.intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>