tools: Add riscv barrier implementation

Many of the other architectures use their custom barrier implementations.
Use the barrier code from the kernel sources to optimize barriers in
tools.

Signed-off-by: Charlie Jenkins <[e-mail]>
Reviewed-by: Andrea Parri <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Palmer Dabbelt <[e-mail]>

tools/arch/riscv/include/asm/barrier.h

···
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copied from the kernel sources to tools/arch/riscv:
+ *
+ * Copyright (C) 2012 ARM Ltd.
+ * Copyright (C) 2013 Regents of the University of California
+ * Copyright (C) 2017 SiFive
+ */
+
+#ifndef _TOOLS_LINUX_ASM_RISCV_BARRIER_H
+#define _TOOLS_LINUX_ASM_RISCV_BARRIER_H
+
+#include <asm/fence.h>
+#include <linux/compiler.h>
+
+/* These barriers need to enforce ordering on both devices and memory. */
+#define mb()		RISCV_FENCE(iorw, iorw)
+#define rmb()		RISCV_FENCE(ir, ir)
+#define wmb()		RISCV_FENCE(ow, ow)
+
+/* These barriers do not need to enforce ordering on devices, just memory. */
+#define smp_mb()	RISCV_FENCE(rw, rw)
+#define smp_rmb()	RISCV_FENCE(r, r)
+#define smp_wmb()	RISCV_FENCE(w, w)
+
+#define smp_store_release(p, v)						\
+do {									\
+	RISCV_FENCE(rw, w);						\
+	WRITE_ONCE(*p, v);						\
+} while (0)
+
+#define smp_load_acquire(p)						\
+({									\
+	typeof(*p) ___p1 = READ_ONCE(*p);				\
+	RISCV_FENCE(r, rw);						\
+	___p1;								\
+})
+
+#endif /* _TOOLS_LINUX_ASM_RISCV_BARRIER_H */

tools/arch/riscv/include/asm/fence.h

···
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copied from the kernel sources to tools/arch/riscv:
+ */
+
+#ifndef _ASM_RISCV_FENCE_H
+#define _ASM_RISCV_FENCE_H
+
+#define RISCV_FENCE_ASM(p, s)		"\tfence " #p "," #s "\n"
+#define RISCV_FENCE(p, s) \
+	({ __asm__ __volatile__ (RISCV_FENCE_ASM(p, s) : : : "memory"); })
+
+#endif	/* _ASM_RISCV_FENCE_H */

tools/include/asm/barrier.h

···
 #include "../../arch/arm64/include/asm/barrier.h"
 #elif defined(__powerpc__)
 #include "../../arch/powerpc/include/asm/barrier.h"
+#elif defined(__riscv)
+#include "../../arch/riscv/include/asm/barrier.h"
 #elif defined(__s390__)
 #include "../../arch/s390/include/asm/barrier.h"
 #elif defined(__sh__)