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>

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 5372e1e..8dabffa 100644 (file)
--- 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

diff --cc arch/arm64/include/asm/atomic.h
Simple merge

diff --cc arch/arm64/include/asm/atomic_ll_sc.h
Simple merge

diff --cc arch/arm64/include/asm/atomic_lse.h
Simple merge

diff --cc arch/arm64/lib/Makefile
Simple merge

diff --cc arch/arm64/lib/atomic_ll_sc.c
Simple merge