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>

arch/arm64/include/asm/atomic.h

@@ -21,6 +21,7 @@
 #define __ASM_ATOMIC_H
 
 #include <linux/compiler.h>
+#include <linux/stringify.h>
 #include <linux/types.h>
 
 #include <asm/barrier.h>
@@ -30,7 +31,15 @@
 
 #ifdef __KERNEL__
 
+#define __ARM64_IN_ATOMIC_IMPL
+
+#ifdef CONFIG_ARM64_LSE_ATOMICS
+#include <asm/atomic_lse.h>
+#else
 #include <asm/atomic_ll_sc.h>
+#endif
+
+#undef __ARM64_IN_ATOMIC_IMPL
 
 /*
  * On ARM, ordinary assignment (str instruction) doesn't clear the local