The CPPR (Current Processor Priority Register) of a XICS interrupt
presentation controller contains a value N, such that only interrupts
with a priority "more favoured" than N will be received by the CPU,
where "more favoured" means "less than". So if the CPPR has the value 5
then only interrupts with a priority of 0-4 inclusive will be received.

In theory the CPPR can support a value of 0 to 255 inclusive.
In practice Linux only uses values of 0, 4, 5 and 0xff. Setting the CPPR
to 0 rejects all interrupts, setting it to 0xff allows all interrupts.
The values 4 and 5 are used to differentiate IPIs from external
interrupts. Setting the CPPR to 5 allows IPIs to be received but not
external interrupts.

The CPPR emulation in the OPAL XICS implementation only directly
supports priorities 0 and 0xff. All other priorities are considered
equivalent, and mapped to a single priority value internally. This means
when using icp-opal we can not allow IPIs but not externals.

This breaks Linux's use of priority values when a CPU is hot unplugged.
After migrating IRQs away from the CPU that is being offlined, we set
the priority to 5, meaning we still want the offline CPU to receive
IPIs. But the effect of the OPAL XICS emulation's use of a single
priority value is that all interrupts are rejected by the CPU. With the
CPU offline, and not receiving IPIs, we may not be able to wake it up to
bring it back online.

The first part of the fix is in icp_opal_set_cpu_priority(). CPPR values
of 0 to 4 inclusive will correctly cause all interrupts to be rejected,
so we pass those CPPR values through to OPAL. However if we are called
with a CPPR of 5 or greater, the caller is expecting to be able to allow
IPIs but not external interrupts. We know this doesn't work, so instead
of rejecting all interrupts we choose the opposite which is to allow all
interrupts. This is still not correct behaviour, but we know for the
only existing caller (xics_migrate_irqs_away()), that it is the better
option.

The other part of the fix is in xics_migrate_irqs_away(). Instead of
setting priority (CPPR) to 0, and then back to 5 before migrating IRQs,
we migrate the IRQs before setting the priority back to 5. This should
have no effect on an ICP backend with a working set_priority(), and on
icp-opal it means we will keep all interrupts blocked until after we've
finished doing the IRQ migration. Additionally we wait for 5ms after
doing the migration to make sure there are no IRQs in flight.

Fixes: d7436188 ("powerpc/xics: Add ICP OPAL backend")
Cc: stable@vger.kernel.org # v4.8+
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Reported-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Tested-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
[mpe: Rewrote comments and change log, change delay to 5ms]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>