hw/misc: Add npcm7xx random number generator

The RNG module returns a byte of randomness when the Data Valid bit is
set.

This implementation ignores the prescaler setting, and loads a new value
into RNGD every time RNGCS is read while the RNG is enabled and random
data is available.

A qtest featuring some simple randomness tests is included.

Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 278 insertions, 0 deletions

diff --git a/tests/qtest/npcm7xx_rng-test.c b/tests/qtest/npcm7xx_rng-test.c
new file mode 100644
index 0000000000..da6e639bf6
--- /dev/null
+++ b/tests/qtest/npcm7xx_rng-test.c
@@ -0,0 +1,278 @@
+/*
+ * QTest testcase for the Nuvoton NPCM7xx Random Number Generator
+ *
+ * Copyright 2020 Google LLC
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ * for more details.
+ */
+
+#include "qemu/osdep.h"
+
+#include <math.h>
+
+#include "libqtest-single.h"
+#include "qemu/bitops.h"
+
+#define RNG_BASE_ADDR   0xf000b000
+
+/* Control and Status Register */
+#define RNGCS   0x00
+# define DVALID     BIT(1)  /* Data Valid */
+# define RNGE       BIT(0)  /* RNG Enable */
+/* Data Register */
+#define RNGD    0x04
+/* Mode Register */
+#define RNGMODE 0x08
+# define ROSEL_NORMAL   (2) /* RNG only works in this mode */
+
+/* Number of bits to collect for randomness tests. */
+#define TEST_INPUT_BITS  (128)
+
+static void rng_writeb(unsigned int offset, uint8_t value)
+{
+    writeb(RNG_BASE_ADDR + offset, value);
+}
+
+static uint8_t rng_readb(unsigned int offset)
+{
+    return readb(RNG_BASE_ADDR + offset);
+}
+
+/* Disable RNG and set normal ring oscillator mode. */
+static void rng_reset(void)
+{
+    rng_writeb(RNGCS, 0);
+    rng_writeb(RNGMODE, ROSEL_NORMAL);
+}
+
+/* Reset RNG and then enable it. */
+static void rng_reset_enable(void)
+{
+    rng_reset();
+    rng_writeb(RNGCS, RNGE);
+}
+
+/* Wait until Data Valid bit is set. */
+static bool rng_wait_ready(void)
+{
+    /* qemu_guest_getrandom may fail. Assume it won't fail 10 times in a row. */
+    int retries = 10;
+
+    while (retries-- > 0) {
+        if (rng_readb(RNGCS) & DVALID) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+/*
+ * Perform a frequency (monobit) test, as defined by NIST SP 800-22, on the
+ * sequence in buf and return the P-value. This represents the probability of a
+ * truly random sequence having the same proportion of zeros and ones as the
+ * sequence in buf.
+ *
+ * An RNG which always returns 0x00 or 0xff, or has some bits stuck at 0 or 1,
+ * will fail this test. However, an RNG which always returns 0x55, 0xf0 or some
+ * other value with an equal number of zeroes and ones will pass.
+ */
+static double calc_monobit_p(const uint8_t *buf, unsigned int len)
+{
+    unsigned int i;
+    double s_obs;
+    int sn = 0;
+
+    for (i = 0; i < len; i++) {
+        /*
+         * Each 1 counts as 1, each 0 counts as -1.
+         * s = cp - (8 - cp) = 2 * cp - 8
+         */
+        sn += 2 * ctpop8(buf[i]) - 8;
+    }
+
+    s_obs = abs(sn) / sqrt(len * BITS_PER_BYTE);
+
+    return erfc(s_obs / sqrt(2));
+}
+
+/*
+ * Perform a runs test, as defined by NIST SP 800-22, and return the P-value.
+ * This represents the probability of a truly random sequence having the same
+ * number of runs (i.e. uninterrupted sequences of identical bits) as the
+ * sequence in buf.
+ */
+static double calc_runs_p(const unsigned long *buf, unsigned int nr_bits)
+{
+    unsigned int j;
+    unsigned int k;
+    int nr_ones = 0;
+    int vn_obs = 0;
+    double pi;
+
+    g_assert(nr_bits % BITS_PER_LONG == 0);
+
+    for (j = 0; j < nr_bits / BITS_PER_LONG; j++) {
+        nr_ones += __builtin_popcountl(buf[j]);
+    }
+    pi = (double)nr_ones / nr_bits;
+
+    for (k = 0; k < nr_bits - 1; k++) {
+        vn_obs += !(test_bit(k, buf) ^ test_bit(k + 1, buf));
+    }
+    vn_obs += 1;
+
+    return erfc(fabs(vn_obs - 2 * nr_bits * pi * (1.0 - pi))
+                / (2 * sqrt(2 * nr_bits) * pi * (1.0 - pi)));
+}
+
+/*
+ * Verifies that DVALID is clear, and RNGD reads zero, when RNGE is cleared,
+ * and DVALID eventually becomes set when RNGE is set.
+ */
+static void test_enable_disable(void)
+{
+    /* Disable: DVALID should not be set, and RNGD should read zero */
+    rng_reset();
+    g_assert_cmphex(rng_readb(RNGCS), ==, 0);
+    g_assert_cmphex(rng_readb(RNGD), ==, 0);
+
+    /* Enable: DVALID should be set, but we can't make assumptions about RNGD */
+    rng_writeb(RNGCS, RNGE);
+    g_assert_true(rng_wait_ready());
+    g_assert_cmphex(rng_readb(RNGCS), ==, DVALID | RNGE);
+
+    /* Disable: DVALID should not be set, and RNGD should read zero */
+    rng_writeb(RNGCS, 0);
+    g_assert_cmphex(rng_readb(RNGCS), ==, 0);
+    g_assert_cmphex(rng_readb(RNGD), ==, 0);
+}
+
+/*
+ * Verifies that the RNG only produces data when RNGMODE is set to 'normal'
+ * ring oscillator mode.
+ */
+static void test_rosel(void)
+{
+    rng_reset_enable();
+    g_assert_true(rng_wait_ready());
+    rng_writeb(RNGMODE, 0);
+    g_assert_false(rng_wait_ready());
+    rng_writeb(RNGMODE, ROSEL_NORMAL);
+    g_assert_true(rng_wait_ready());
+    rng_writeb(RNGMODE, 0);
+    g_assert_false(rng_wait_ready());
+}
+
+/*
+ * Verifies that a continuous sequence of bits collected after enabling the RNG
+ * satisfies a monobit test.
+ */
+static void test_continuous_monobit(void)
+{
+    uint8_t buf[TEST_INPUT_BITS / BITS_PER_BYTE];
+    unsigned int i;
+
+    rng_reset_enable();
+    for (i = 0; i < sizeof(buf); i++) {
+        g_assert_true(rng_wait_ready());
+        buf[i] = rng_readb(RNGD);
+    }
+
+    g_assert_cmpfloat(calc_monobit_p(buf, sizeof(buf)), >, 0.01);
+}
+
+/*
+ * Verifies that a continuous sequence of bits collected after enabling the RNG
+ * satisfies a runs test.
+ */
+static void test_continuous_runs(void)
+{
+    union {
+        unsigned long l[TEST_INPUT_BITS / BITS_PER_LONG];
+        uint8_t c[TEST_INPUT_BITS / BITS_PER_BYTE];
+    } buf;
+    unsigned int i;
+
+    rng_reset_enable();
+    for (i = 0; i < sizeof(buf); i++) {
+        g_assert_true(rng_wait_ready());
+        buf.c[i] = rng_readb(RNGD);
+    }
+
+    g_assert_cmpfloat(calc_runs_p(buf.l, sizeof(buf) * BITS_PER_BYTE), >, 0.01);
+}
+
+/*
+ * Verifies that the first data byte collected after enabling the RNG satisfies
+ * a monobit test.
+ */
+static void test_first_byte_monobit(void)
+{
+    /* Enable, collect one byte, disable. Repeat until we have 100 bits. */
+    uint8_t buf[TEST_INPUT_BITS / BITS_PER_BYTE];
+    unsigned int i;
+
+    rng_reset();
+    for (i = 0; i < sizeof(buf); i++) {
+        rng_writeb(RNGCS, RNGE);
+        g_assert_true(rng_wait_ready());
+        buf[i] = rng_readb(RNGD);
+        rng_writeb(RNGCS, 0);
+    }
+
+    g_assert_cmpfloat(calc_monobit_p(buf, sizeof(buf)), >, 0.01);
+}
+
+/*
+ * Verifies that the first data byte collected after enabling the RNG satisfies
+ * a runs test.
+ */
+static void test_first_byte_runs(void)
+{
+    /* Enable, collect one byte, disable. Repeat until we have 100 bits. */
+    union {
+        unsigned long l[TEST_INPUT_BITS / BITS_PER_LONG];
+        uint8_t c[TEST_INPUT_BITS / BITS_PER_BYTE];
+    } buf;
+    unsigned int i;
+
+    rng_reset();
+    for (i = 0; i < sizeof(buf); i++) {
+        rng_writeb(RNGCS, RNGE);
+        g_assert_true(rng_wait_ready());
+        buf.c[i] = rng_readb(RNGD);
+        rng_writeb(RNGCS, 0);
+    }
+
+    g_assert_cmpfloat(calc_runs_p(buf.l, sizeof(buf) * BITS_PER_BYTE), >, 0.01);
+}
+
+int main(int argc, char **argv)
+{
+    int ret;
+
+    g_test_init(&argc, &argv, NULL);
+    g_test_set_nonfatal_assertions();
+
+    qtest_add_func("npcm7xx_rng/enable_disable", test_enable_disable);
+    qtest_add_func("npcm7xx_rng/rosel", test_rosel);
+    qtest_add_func("npcm7xx_rng/continuous/monobit", test_continuous_monobit);
+    qtest_add_func("npcm7xx_rng/continuous/runs", test_continuous_runs);
+    qtest_add_func("npcm7xx_rng/first_byte/monobit", test_first_byte_monobit);
+    qtest_add_func("npcm7xx_rng/first_byte/runs", test_first_byte_runs);
+
+    qtest_start("-machine npcm750-evb");
+    ret = g_test_run();
+    qtest_end();
+
+    return ret;
+}