Quoting Douglas Anderson (2023-08-24 08:30:29)

From: Mark Rutland <mark.rutland@arm.com>

To enable NMI backtrace and KGDB's NMI cpu roundup, we need to free up
at least one dedicated IPI.

On arm64 the IPI_WAKEUP IPI is only used for the ACPI parking protocol,
which itself is only used on some very early ARMv8 systems which
couldn't implement PSCI.

Remove the IPI_WAKEUP IPI, and rely on the IPI_RESCHEDULE IPI to wake
CPUs from the parked state. This will cause a tiny amonut of redundant
work to check the thread flags, but this is miniscule in relation to the
cost of taking and handling the IPI in the first place. We can safely
handle redundant IPI_RESCHEDULE IPIs, so there should be no functional
impact as a result of this change.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Sumit Garg <redacted>
Cc: Will Deacon <will@kernel.org>
---

Reviewed-by: Stephen Boyd <redacted>

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