1 files changed, 62 insertions, 0 deletions

diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
index 88461f09cc86..9098083869c8 100644
--- a/tools/include/linux/compiler.h
+++ b/tools/include/linux/compiler.h
@@ -1,6 +1,10 @@
 #ifndef _TOOLS_LINUX_COMPILER_H_
 #define _TOOLS_LINUX_COMPILER_H_
 
+/* Optimization barrier */
+/* The "volatile" is due to gcc bugs */
+#define barrier() __asm__ __volatile__("": : :"memory")
+
 #ifndef __always_inline
 # define __always_inline        inline __attribute__((always_inline))
 #endif
@@ -37,4 +41,62 @@
 
 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
 
+#include <linux/types.h>
+
+static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
+{
+        switch (size) {
+        case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
+        case 2: *(__u16 *)res = *(volatile __u16 *)p; break;
+        case 4: *(__u32 *)res = *(volatile __u32 *)p; break;
+        case 8: *(__u64 *)res = *(volatile __u64 *)p; break;
+        default:
+                barrier();
+                __builtin_memcpy((void *)res, (const void *)p, size);
+                barrier();
+        }
+}
+
+static __always_inline void __write_once_size(volatile void *p, void *res, int size)
+{
+        switch (size) {
+        case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
+        case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
+        case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
+        case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
+        default:
+                barrier();
+                __builtin_memcpy((void *)p, (const void *)res, size);
+                barrier();
+        }
+}
+
+/*
+ * Prevent the compiler from merging or refetching reads or writes. The
+ * compiler is also forbidden from reordering successive instances of
+ * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
+ * compiler is aware of some particular ordering.  One way to make the
+ * compiler aware of ordering is to put the two invocations of READ_ONCE,
+ * WRITE_ONCE or ACCESS_ONCE() in different C statements.
+ *
+ * In contrast to ACCESS_ONCE these two macros will also work on aggregate
+ * data types like structs or unions. If the size of the accessed data
+ * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
+ * READ_ONCE() and WRITE_ONCE()  will fall back to memcpy and print a
+ * compile-time warning.
+ *
+ * Their two major use cases are: (1) Mediating communication between
+ * process-level code and irq/NMI handlers, all running on the same CPU,
+ * and (2) Ensuring that the compiler does not  fold, spindle, or otherwise
+ * mutilate accesses that either do not require ordering or that interact
+ * with an explicit memory barrier or atomic instruction that provides the
+ * required ordering.
+ */
+
+#define READ_ONCE(x) \
+        ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
+
+#define WRITE_ONCE(x, val) \
+        ({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
+
 #endif /* _TOOLS_LINUX_COMPILER_H */