This calls into the Arm Trusted Firmware to enable and disable the
workaround for the Speculative Store Bypass Disable (SSBD) issue, also
known as Spectre Variant 4.
As this may have a large performance overhead, and how exploitable SSBD is
is unknown we follow the Linux lead of allowing the administrator to select
between always on, always off, or only enabled in the kernel, with the
latter being the default.
PR: 228955
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D15819
to call into the firmware in a similar way to the existing PSCI, and used
PSCI to detect when SMCCC is enabled.
There is a function ID space we can use. Currently we only support 3
functions in the ARM Architecture Calls region, however it is expected we
will expend these in the future.
Sponsored by: DARPA, AFRL
