arm64: introduce CONFIG_ARM64_LSE_ATOMICS as fallback to ll/sc atomics

In order to patch in the new atomic instructions at runtime, we need to
generate wrappers around the out-of-line exclusive load/store atomics.

This patch adds a new Kconfig option, CONFIG_ARM64_LSE_ATOMICS. which
causes our atomic functions to branch to the out-of-line ll/sc
implementations. To avoid the register spill overhead of the PCS, the
out-of-line functions are compiled with specific compiler flags to
force out-of-line save/restore of any registers that are usually
caller-saved.

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

1 files changed, 12 insertions, 0 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 5372e1e..8dabffa 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -618,6 +618,18 @@ config ARM64_PAN
 	 The feature is detected at runtime, and will remain as a 'nop'
 	 instruction if the cpu does not implement the feature.
 
+config ARM64_LSE_ATOMICS
+	bool "ARMv8.1 atomic instructions"
+	help
+	  As part of the Large System Extensions, ARMv8.1 introduces new
+	  atomic instructions that are designed specifically to scale in
+	  very large systems.
+
+	  Say Y here to make use of these instructions for the in-kernel
+	  atomic routines. This incurs a small overhead on CPUs that do
+	  not support these instructions and requires the kernel to be
+	  built with binutils >= 2.25.
+
 menuconfig ARMV8_DEPRECATED
 	bool "Emulate deprecated/obsolete ARMv8 instructions"
 	depends on COMPAT