plat/arm: juno: Condition Juno entropy source with CRC instructions

The Juno Trusted Entropy Source has a bias, which makes the generated
raw numbers fail a FIPS 140-2 statistic test.

To improve the quality of the numbers, we can use the CPU's CRC
instructions, which do a decent job on conditioning the bits.

This adds a *very* simple version of arm_acle.h, which is typically
provided by the compiler, and contains the CRC instrinsics definitions
we need. We need the original version by using -nostdinc.

Change-Id: I83d3e6902d6a1164aacd5060ac13a38f0057bd1a
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

include/lib/libc/arm_acle.h

+/*
+ * Copyright (c) 2021 ARM Limited
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * The definitions below are a subset of what we would normally get by using
+ * the compiler's version of arm_acle.h. We can't use that directly because
+ * we specify -nostdinc in the Makefiles.
+ *
+ * We just define the functions we need so far.
+ */
+
+#ifndef ARM_ACLE_H
+#define ARM_ACLE_H
+
+#if !defined(__aarch64__) || defined(__clang__)
+#	define __crc32w __builtin_arm_crc32w
+#else
+#	define __crc32w __builtin_aarch64_crc32w
+#endif
+
+#endif	/* ARM_ACLE_H */

plat/arm/board/juno/juno_trng.c

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
+#include <arm_acle.h>
 #include <assert.h>
 #include <stdbool.h>
 #include <stdint.h>
@@ -35,6 +36,8 @@ static bool output_valid(void)
 	return false; /* No output data available. */
 }
 
+static uint32_t crc_value = ~0U;
+
 /*
  * This function fills `buf` with 8 bytes of entropy.
  * It uses the Trusted Entropy Source peripheral on Juno.
@@ -69,14 +72,14 @@ bool juno_getentropy(uint64_t *buf)
 			return false;
 	}
 
-	/* XOR each two 32-bit registers together, combine the pairs */
-	ret = mmio_read_32(TRNG_BASE + 0);
-	ret ^= mmio_read_32(TRNG_BASE + 4);
-	ret <<= 32;
+	/* CRC each two 32-bit registers together, combine the pairs */
+	crc_value = __crc32w(crc_value, mmio_read_32(TRNG_BASE + 0));
+	crc_value = __crc32w(crc_value, mmio_read_32(TRNG_BASE + 4));
+	ret = (uint64_t)crc_value << 32;
 
-	ret |= mmio_read_32(TRNG_BASE + 8);
-	ret ^= mmio_read_32(TRNG_BASE + 12);
-	*buf = ret;
+	crc_value = __crc32w(crc_value, mmio_read_32(TRNG_BASE + 8));
+	crc_value = __crc32w(crc_value, mmio_read_32(TRNG_BASE + 12));
+	*buf = ret | crc_value;
 
 	/* Acknowledge current cycle, clear output registers. */
 	mmio_write_32(TRNG_BASE + TRNG_STATUS, 1);

plat/arm/board/juno/platform.mk

@@ -164,6 +164,12 @@ ifeq (${ALLOW_RO_XLAT_TABLES}, 1)
     endif
 endif
 
+BL1_CPPFLAGS += -march=armv8-a+crc
+BL2_CPPFLAGS += -march=armv8-a+crc
+BL2U_CPPFLAGS += -march=armv8-a+crc
+BL31_CPPFLAGS += -march=armv8-a+crc
+BL32_CPPFLAGS += -march=armv8-a+crc
+
 # Add the FDT_SOURCES and options for Dynamic Config
 FDT_SOURCES		+=	plat/arm/board/juno/fdts/${PLAT}_fw_config.dts	\
 				plat/arm/board/juno/fdts/${PLAT}_tb_fw_config.dts