Utility for using the RDRAND instruction

On some x86_64 hardware the RDRAND instruction returns 64bits of entropy
from an on-chip random number generator. RDRAND is not recommended as an
exclusive source of entropy for cryptographic applications.

https://en.wikipedia.org/wiki/RdRand

include/spec/x86_64/world/rdrand.h

@@ -0,0 +1,56 @@
+/*
+ * \brief   Utility for reading hardware RNG
+ * \date    2019-02-05
+ * \author  Emery Hemingway
+ */
+
+/*
+ * Copyright (C) 2019 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#ifndef _INCLUDE__SPEC__X86_64__OS__RDRAND_H_
+#define _INCLUDE__SPEC__X86_64__OS__RDRAND_H_
+
+#include <base/stdint.h>
+#include <base/log.h>
+
+namespace Genode { namespace Rdrand {
+
+	bool supported()
+	{
+		unsigned int info[4] = {0, 0, 0, 0};
+
+		asm volatile("cpuid":"=a"(info[0]),"=b"(info[1]),"=c"(info[2]),"=d"(info[3]):"a"(1),"c"(0));
+
+		enum { RDRAND_MASK = 0x40000000 };
+		return (info[2] & RDRAND_MASK);
+	}
+
+#	define _rdrand_step(x) \
+	({ \
+		unsigned char err; \
+		asm volatile("rdrand %0; setc %1":"=r"(x), "=qm"(err)); \
+		err; \
+	}) \
+
+	uint64_t random64()
+	{
+		uint64_t result = 0;
+
+		enum { RETRIES = 8 };
+		for (int i = 0; i < RETRIES; i++)
+			if (_rdrand_step(result))
+				return result;
+
+		error("RDRAND failure");
+		throw Exception();
+	}
+
+#	undef _rdrand_step
+
+} }
+
+#endif /* _INCLUDE__SPEC__X86_64__OS__RDRAND_H_ */

include/world/rdrand.h

@@ -0,0 +1,33 @@
+/*
+ * \brief   Utility for reading hardware RNG
+ * \date    2019-02-05
+ * \author  Emery Hemingway
+ */
+
+/*
+ * Copyright (C) 2019 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#ifndef _INCLUDE__OS__RDRAND_H_
+#define _INCLUDE__OS__RDRAND_H_
+
+#include <base/stdint.h>
+#include <base/log.h>
+
+namespace Genode { namespace Rdrand {
+
+	constexpr
+	bool supported() { return false; }
+
+	uint64_t random64()
+	{
+		error("RDRAND not available on this architecture");
+		throw Exception();
+	}
+
+} }
+
+#endif /* _INCLUDE__OS__RDRAND_H_ */