diff --git a/tests/test-cutils.c b/tests/test-cutils.c
index 1aa8351520..6fa9f88488 100644
--- a/tests/test-cutils.c
+++ b/tests/test-cutils.c
@@ -1970,40 +1970,25 @@ static void test_qemu_strtosz_simple(void)
     g_assert_cmpint(err, ==, 0);
     g_assert_cmpint(res, ==, 12345);
 
-    /* Note: precision is 53 bits since we're parsing with strtod() */
-
-    str = "9007199254740991"; /* 2^53-1 */
-    err = qemu_strtosz(str, &endptr, &res);
-    g_assert_cmpint(err, ==, 0);
-    g_assert_cmpint(res, ==, 0x1fffffffffffff);
-    g_assert(endptr == str + 16);
-
-    str = "9007199254740992"; /* 2^53 */
-    err = qemu_strtosz(str, &endptr, &res);
-    g_assert_cmpint(err, ==, 0);
-    g_assert_cmpint(res, ==, 0x20000000000000);
-    g_assert(endptr == str + 16);
+    /* Note: precision is 64 bits (UINT64_MAX) */
 
     str = "9007199254740993"; /* 2^53+1 */
     err = qemu_strtosz(str, &endptr, &res);
     g_assert_cmpint(err, ==, 0);
-    g_assert_cmpint(res, ==, 0x20000000000000); /* rounded to 53 bits */
+    g_assert_cmpint(res, ==, 0x20000000000001);
     g_assert(endptr == str + 16);
 
-    str = "18446744073709549568"; /* 0xfffffffffffff800 (53 msbs set) */
+    str = "18446744073709550591"; /* 0xfffffffffffffbff */
     err = qemu_strtosz(str, &endptr, &res);
     g_assert_cmpint(err, ==, 0);
-    g_assert_cmpint(res, ==, 0xfffffffffffff800);
+    g_assert_cmpint(res, ==, 0xfffffffffffffbff);
     g_assert(endptr == str + 20);
 
-    str = "18446744073709550591"; /* 0xfffffffffffffbff */
+    str = "18446744073709551615"; /* 2^64-1 (UINT64_MAX) */
     err = qemu_strtosz(str, &endptr, &res);
     g_assert_cmpint(err, ==, 0);
-    g_assert_cmpint(res, ==, 0xfffffffffffff800); /* rounded to 53 bits */
+    g_assert_cmpint(res, ==, 0xffffffffffffffff);
     g_assert(endptr == str + 20);
-
-    /* 0x7ffffffffffffe00..0x7fffffffffffffff get rounded to
-     * 0x8000000000000000, thus -ERANGE; see test_qemu_strtosz_erange() */
 }
 
 static void test_qemu_strtosz_units(void)
@@ -2145,20 +2130,20 @@ static void test_qemu_strtosz_erange(void)
     g_assert_cmpint(err, ==, -ERANGE);
     g_assert(endptr == str + 2);
 
-    str = "18446744073709550592"; /* 0xfffffffffffffc00 */
+    str = "18446744073709551616"; /* 2^64 */
     err = qemu_strtosz(str, &endptr, &res);
     g_assert_cmpint(err, ==, -ERANGE);
     g_assert(endptr == str + 20);
 
-    str = "18446744073709551615"; /* 2^64-1 */
+    str = "1.7976931348623158e+308"; /* DBL_MAX */
     err = qemu_strtosz(str, &endptr, &res);
     g_assert_cmpint(err, ==, -ERANGE);
-    g_assert(endptr == str + 20);
+    g_assert(endptr == str + 23);
 
-    str = "18446744073709551616"; /* 2^64 */
+    str = "2.2250738585072014e-308"; /* DBL_MIN */
     err = qemu_strtosz(str, &endptr, &res);
     g_assert_cmpint(err, ==, -ERANGE);
-    g_assert(endptr == str + 20);
+    g_assert(endptr == str + 23);
 
     str = "20E";
     err = qemu_strtosz(str, &endptr, &res);
diff --git a/tests/test-keyval.c b/tests/test-keyval.c
index 09b0ae3c68..fad941fcb8 100644
--- a/tests/test-keyval.c
+++ b/tests/test-keyval.c
@@ -383,59 +383,26 @@ static void test_keyval_visit_size(void)
     visit_end_struct(v, NULL);
     visit_free(v);
 
-    /* Note: precision is 53 bits since we're parsing with strtod() */
+    /* Note: precision is 64 bits (UINT64_MAX) */
 
-    /* Around limit of precision: 2^53-1, 2^53, 2^53+1 */
-    qdict = keyval_parse("sz1=9007199254740991,"
-                         "sz2=9007199254740992,"
-                         "sz3=9007199254740993",
+    /* Around limit of precision: UINT64_MAX - 1, UINT64_MAX */
+    qdict = keyval_parse("sz1=18446744073709551614,"
+                         "sz2=18446744073709551615",
                          NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
     visit_type_size(v, "sz1", &sz, &error_abort);
-    g_assert_cmphex(sz, ==, 0x1fffffffffffff);
+    g_assert_cmphex(sz, ==, 0xfffffffffffffffe);
     visit_type_size(v, "sz2", &sz, &error_abort);
-    g_assert_cmphex(sz, ==, 0x20000000000000);
-    visit_type_size(v, "sz3", &sz, &error_abort);
-    g_assert_cmphex(sz, ==, 0x20000000000000);
-    visit_check_struct(v, &error_abort);
-    visit_end_struct(v, NULL);
-    visit_free(v);
-
-    /* Close to signed upper limit 0x7ffffffffffffc00 (53 msbs set) */
-    qdict = keyval_parse("sz1=9223372036854774784," /* 7ffffffffffffc00 */
-                         "sz2=9223372036854775295", /* 7ffffffffffffdff */
-                         NULL, &error_abort);
-    v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
-    qobject_unref(qdict);
-    visit_start_struct(v, NULL, NULL, 0, &error_abort);
-    visit_type_size(v, "sz1", &sz, &error_abort);
-    g_assert_cmphex(sz, ==, 0x7ffffffffffffc00);
-    visit_type_size(v, "sz2", &sz, &error_abort);
-    g_assert_cmphex(sz, ==, 0x7ffffffffffffc00);
-    visit_check_struct(v, &error_abort);
-    visit_end_struct(v, NULL);
-    visit_free(v);
-
-    /* Close to actual upper limit 0xfffffffffffff800 (53 msbs set) */
-    qdict = keyval_parse("sz1=18446744073709549568," /* fffffffffffff800 */
-                         "sz2=18446744073709550591", /* fffffffffffffbff */
-                         NULL, &error_abort);
-    v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
-    qobject_unref(qdict);
-    visit_start_struct(v, NULL, NULL, 0, &error_abort);
-    visit_type_size(v, "sz1", &sz, &error_abort);
-    g_assert_cmphex(sz, ==, 0xfffffffffffff800);
-    visit_type_size(v, "sz2", &sz, &error_abort);
-    g_assert_cmphex(sz, ==, 0xfffffffffffff800);
+    g_assert_cmphex(sz, ==, 0xffffffffffffffff);
     visit_check_struct(v, &error_abort);
     visit_end_struct(v, NULL);
     visit_free(v);
 
     /* Beyond limits */
     qdict = keyval_parse("sz1=-1,"
-                         "sz2=18446744073709550592", /* fffffffffffffc00 */
+                         "sz2=18446744073709551616", /* 2^64 */
                          NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
diff --git a/tests/test-qemu-opts.c b/tests/test-qemu-opts.c
index ef96e84aed..3a8b8c0168 100644
--- a/tests/test-qemu-opts.c
+++ b/tests/test-qemu-opts.c
@@ -650,50 +650,25 @@ static void test_opts_parse_size(void)
     g_assert_cmpuint(opts_count(opts), ==, 1);
     g_assert_cmpuint(qemu_opt_get_size(opts, "size1", 1), ==, 0);
 
-    /* Note: precision is 53 bits since we're parsing with strtod() */
+    /* Note: precision is 64 bits (UINT64_MAX) */
 
-    /* Around limit of precision: 2^53-1, 2^53, 2^54 */
+    /* Around limit of precision: UINT64_MAX - 1, UINT64_MAX */
     opts = qemu_opts_parse(&opts_list_02,
-                           "size1=9007199254740991,"
-                           "size2=9007199254740992,"
-                           "size3=9007199254740993",
-                           false, &error_abort);
-    g_assert_cmpuint(opts_count(opts), ==, 3);
-    g_assert_cmphex(qemu_opt_get_size(opts, "size1", 1),
-                     ==, 0x1fffffffffffff);
-    g_assert_cmphex(qemu_opt_get_size(opts, "size2", 1),
-                     ==, 0x20000000000000);
-    g_assert_cmphex(qemu_opt_get_size(opts, "size3", 1),
-                     ==, 0x20000000000000);
-
-    /* Close to signed upper limit 0x7ffffffffffffc00 (53 msbs set) */
-    opts = qemu_opts_parse(&opts_list_02,
-                           "size1=9223372036854774784," /* 7ffffffffffffc00 */
-                           "size2=9223372036854775295", /* 7ffffffffffffdff */
-                           false, &error_abort);
-    g_assert_cmpuint(opts_count(opts), ==, 2);
-    g_assert_cmphex(qemu_opt_get_size(opts, "size1", 1),
-                     ==, 0x7ffffffffffffc00);
-    g_assert_cmphex(qemu_opt_get_size(opts, "size2", 1),
-                     ==, 0x7ffffffffffffc00);
-
-    /* Close to actual upper limit 0xfffffffffffff800 (53 msbs set) */
-    opts = qemu_opts_parse(&opts_list_02,
-                           "size1=18446744073709549568," /* fffffffffffff800 */
-                           "size2=18446744073709550591", /* fffffffffffffbff */
+                           "size1=18446744073709551614,"
+                           "size2=18446744073709551615",
                            false, &error_abort);
     g_assert_cmpuint(opts_count(opts), ==, 2);
     g_assert_cmphex(qemu_opt_get_size(opts, "size1", 1),
-                     ==, 0xfffffffffffff800);
+                     ==, 0xfffffffffffffffe);
     g_assert_cmphex(qemu_opt_get_size(opts, "size2", 1),
-                     ==, 0xfffffffffffff800);
+                     ==, 0xffffffffffffffff);
 
     /* Beyond limits */
     opts = qemu_opts_parse(&opts_list_02, "size1=-1", false, &err);
     error_free_or_abort(&err);
     g_assert(!opts);
     opts = qemu_opts_parse(&opts_list_02,
-                           "size1=18446744073709550592", /* fffffffffffffc00 */
+                           "size1=18446744073709551616", /* 2^64 */
                            false, &err);
     error_free_or_abort(&err);
     g_assert(!opts);
diff --git a/util/cutils.c b/util/cutils.c
index 77acadc70a..7c94ff2f0b 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -212,19 +212,39 @@ static int do_strtosz(const char *nptr, const char **end,
                       const char default_suffix, int64_t unit,
                       uint64_t *result)
 {
-    int retval;
-    const char *endptr;
+    int retval, retd, retu;
+    const char *endptr, *suffixd, *suffixu;
     unsigned char c;
     int mul_required = 0;
-    double val, mul, integral, fraction;
+    bool use_strtod;
+    uint64_t valu;
+    int64_t mul;
+    double vald, integral, fraction;
+
+    /*
+     * Parse @nptr both as a double and as a uint64_t, then use the method
+     * which consumes more characters.
+     */
+    retd = qemu_strtod_finite(nptr, &suffixd, &vald);
+    retu = qemu_strtou64(nptr, &suffixu, 0, &valu);
+    use_strtod = strlen(suffixd) < strlen(suffixu);
+
+    if (use_strtod) {
+        endptr = suffixd;
+        retval = retd;
+    } else {
+        endptr = suffixu;
+        retval = retu;
+    }
 
-    retval = qemu_strtod_finite(nptr, &endptr, &val);
     if (retval) {
         goto out;
     }
-    fraction = modf(val, &integral);
-    if (fraction != 0) {
-        mul_required = 1;
+    if (use_strtod) {
+        fraction = modf(vald, &integral);
+        if (fraction != 0) {
+            mul_required = 1;
+        }
     }
     c = *endptr;
     mul = suffix_mul(c, unit);
@@ -238,17 +258,30 @@ static int do_strtosz(const char *nptr, const char **end,
         retval = -EINVAL;
         goto out;
     }
-    /*
-     * Values near UINT64_MAX overflow to 2**64 when converting to double
-     * precision.  Compare against the maximum representable double precision
-     * value below 2**64, computed as "the next value after 2**64 (0x1p64) in
-     * the direction of 0".
-     */
-    if ((val * mul > nextafter(0x1p64, 0)) || val < 0) {
-        retval = -ERANGE;
-        goto out;
+
+    if (use_strtod) {
+        /*
+         * Values near UINT64_MAX overflow to 2**64 when converting to double
+         * precision. Compare against the maximum representable double precision
+         * value below 2**64, computed as "the next value after 2**64 (0x1p64)
+         * in the direction of 0".
+         */
+        if ((vald * mul > nextafter(0x1p64, 0)) || vald < 0) {
+            retval = -ERANGE;
+            goto out;
+        }
+        *result = vald * mul;
+    } else {
+        /* Reject negative input and overflow output */
+        while (qemu_isspace(*nptr)) {
+            nptr++;
+        }
+        if (*nptr == '-' || UINT64_MAX / mul < valu) {
+            retval = -ERANGE;
+            goto out;
+        }
+        *result = valu * mul;
     }
-    *result = val * mul;
     retval = 0;
 
 out:
