arm64: Document sysctls for emulated deprecated instructions

We have support for emulating a number of deprecated instructions in the
kernel with individual Kconfig options enabling this support per
instruction. In addition to the Kconfig options we also provide runtime
control via sysctls but this is not currently mentioned in the Kconfig so
not very discoverable for users. This is particularly important for
SWP/SWPB since this is disabled by default at runtime and must be enabled
via the sysctl, causing considerable frustration for users who have enabled
the config option and are then confused to find that the instruction is
still faulting.

Add a reference to the sysctls in the help text for each of the config
options, noting that SWP/SWPB is disabled by default, to improve the
user experience.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200625131507.32334-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 66dc41fd49f23b6d67ed218690b49ef5bb1ac119..6c560caf9503d9293f87ea37041ce9dea756f304 100644 (file)
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1327,6 +1327,8 @@ config SWP_EMULATION
          ARMv8 obsoletes the use of A32 SWP/SWPB instructions such that
          they are always undefined. Say Y here to enable software
          emulation of these instructions for userspace using LDXR/STXR.
+         This feature can be controlled at runtime with the abi.swp
+         sysctl which is disabled by default.
 
          In some older versions of glibc [<=2.8] SWP is used during futex
          trylock() operations with the assumption that the code will not
@@ -1353,7 +1355,8 @@ config CP15_BARRIER_EMULATION
          Say Y here to enable software emulation of these
          instructions for AArch32 userspace code. When this option is
          enabled, CP15 barrier usage is traced which can help
-         identify software that needs updating.
+         identify software that needs updating. This feature can be
+         controlled at runtime with the abi.cp15_barrier sysctl.
 
          If unsure, say Y
 
@@ -1364,7 +1367,8 @@ config SETEND_EMULATION
          AArch32 EL0, and is deprecated in ARMv8.
 
          Say Y here to enable software emulation of the instruction
-         for AArch32 userspace code.
+         for AArch32 userspace code. This feature can be controlled
+         at runtime with the abi.setend sysctl.
 
          Note: All the cpus on the system must have mixed endian support at EL0
          for this feature to be enabled. If a new CPU - which doesn't support mixed