drivers/firmware: add SDEI cross-CPU NMI service for arm64

Deliver an NMI-like event to an interrupt-masked arm64 CPU via the
standard SDEI software-signalled event (event 0), without the pseudo-NMI
hot-path cost: register a handler for event 0 and poke a target with
sdei_event_signal(0, mpidr).

First user is arch_trigger_cpumask_backtrace() (sysrq-l, RCU stalls,
hung-task/soft-lockup dumps), which otherwise rides an IPI that can't
reach a masked CPU. Falls back to the IPI path when SDEI is absent; no
watchdog backend yet, so the stock detector is untouched.

Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Yin Fengwei <fengwei_yin@linux.alibaba.com>
Signed-off-by: Will Deacon <will@kernel.org>

0 files changed, 0 insertions, 0 deletions