compiler_types: Introduce the Clang __preserve_most function attribute

[platform/kernel/linux-rpi.git] / include / linux / compiler_types.h

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 547ea1f..c523c66 100644 (file)
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -106,6 +106,34 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 #define __cold
 #endif
 
+/*
+ * On x86-64 and arm64 targets, __preserve_most changes the calling convention
+ * of a function to make the code in the caller as unintrusive as possible. This
+ * convention behaves identically to the C calling convention on how arguments
+ * and return values are passed, but uses a different set of caller- and callee-
+ * saved registers.
+ *
+ * The purpose is to alleviates the burden of saving and recovering a large
+ * register set before and after the call in the caller.  This is beneficial for
+ * rarely taken slow paths, such as error-reporting functions that may be called
+ * from hot paths.
+ *
+ * Note: This may conflict with instrumentation inserted on function entry which
+ * does not use __preserve_most or equivalent convention (if in assembly). Since
+ * function tracing assumes the normal C calling convention, where the attribute
+ * is supported, __preserve_most implies notrace.  It is recommended to restrict
+ * use of the attribute to functions that should or already disable tracing.
+ *
+ * Optional: not supported by gcc.
+ *
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#preserve-most
+ */
+#if __has_attribute(__preserve_most__) && (defined(CONFIG_X86_64) || defined(CONFIG_ARM64))
+# define __preserve_most notrace __attribute__((__preserve_most__))
+#else
+# define __preserve_most
+#endif
+
 /* Builtins */
 
 /*