arm64: Port SWP/SWPB emulation support from arm

The SWP instruction was deprecated in the ARMv6 architecture. The
ARMv7 multiprocessing extensions mandate that SWP/SWPB instructions
are treated as undefined from reset, with the ability to enable them
through the System Control Register SW bit. With ARMv8, the option to
enable these instructions through System Control Register was dropped
as well.

To support legacy applications using these instructions, port the
emulation of the SWP and SWPB instructions from the arm port to arm64.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index aa8f4bea3738..2b6213840ec8 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -180,6 +180,27 @@ menuconfig ARMV8_DEPRECATED
 
 if ARMV8_DEPRECATED
 
+config SWP_EMULATION
+        bool "Emulate SWP/SWPB instructions"
+        help
+          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.
+
+          In some older versions of glibc [<=2.8] SWP is used during futex
+          trylock() operations with the assumption that the code will not
+          be preempted. This invalid assumption may be more likely to fail
+          with SWP emulation enabled, leading to deadlock of the user
+          application.
+
+          NOTE: when accessing uncached shared regions, LDXR/STXR rely
+          on an external transaction monitoring block called a global
+          monitor to maintain update atomicity. If your system does not
+          implement a global monitor, this option can cause programs that
+          perform SWP operations to uncached memory to deadlock.
+
+          If unsure, say Y
+
 endif
 
 endmenu