diff --git a/doc/source/user/flavors.rst b/doc/source/user/flavors.rst
index 5d964493c6a2..30704be4a024 100644
--- a/doc/source/user/flavors.rst
+++ b/doc/source/user/flavors.rst
@@ -625,9 +625,11 @@ CPU real-time policy
     - ``yes``: The guest vCPUs will have a real-time policy
 
   CPU-REALTIME-MASK (coremask):
-    A coremask indicating which vCPUs **will not** have a real-time policy. This
-    should start with a ``^``. For example, a value of ``^0-1`` indicates that
-    all vCPUs *except* vCPUs ``0`` and ``1`` will have a real-time policy.
+    A coremask indicating which vCPUs **will** or, if starting with a ``^``,
+    **will not** have a real-time policy. For example, a value of ``0-5``
+    indicates that vCPUs ``0`` to ``5`` will have a real-time policy.
+    Conversely, a value of ``^0-1`` indicates that all vCPUs *except* vCPUs
+    ``0`` and ``1`` will have a real-time policy.
 
   .. note::
 
@@ -641,6 +643,12 @@ CPU real-time policy
      to omit this when an emulator thread policy is configured using the
      ``hw:emulator_threads_policy`` extra spec.
 
+  .. versionchanged:: 22.0.0 (Victoria)
+
+     Previously, the leading carat was necessary and omitting it would be
+     equivalent to not setting the mask, resulting in a failure to spawn
+     the instance.
+
 .. _extra-specs-emulator-threads-policy:
 
 Emulator threads policy
diff --git a/nova/api/validation/extra_specs/hw.py b/nova/api/validation/extra_specs/hw.py
index b9d4def4a982..fd4aae4cb4cf 100644
--- a/nova/api/validation/extra_specs/hw.py
+++ b/nova/api/validation/extra_specs/hw.py
@@ -36,8 +36,7 @@ realtime_validators = [
         ),
         value={
             'type': str,
-            # NOTE(stephenfin): Yes, these things *have* to start with '^'
-            'pattern': r'\^\d+((-\d+)?(,\^?\d+(-\d+)?)?)*',
+            'pattern': r'(\^)?\d+((-\d+)?(,\^?\d+(-\d+)?)?)*',
         },
     ),
 ]
diff --git a/nova/tests/unit/api/validation/extra_specs/test_validators.py b/nova/tests/unit/api/validation/extra_specs/test_validators.py
index 2670c5c5acfb..6f11673fd034 100644
--- a/nova/tests/unit/api/validation/extra_specs/test_validators.py
+++ b/nova/tests/unit/api/validation/extra_specs/test_validators.py
@@ -59,6 +59,7 @@ class TestValidators(test.NoDBTestCase):
     def test_value__str(self):
         valid_specs = (
             # patterns
+            ('hw:cpu_realtime_mask', '0'),
             ('hw:cpu_realtime_mask', '^0'),
             ('hw:cpu_realtime_mask', '^0,2-3,1'),
             ('hw:mem_page_size', 'large'),
@@ -74,7 +75,7 @@ class TestValidators(test.NoDBTestCase):
 
         invalid_specs = (
             # patterns
-            ('hw:cpu_realtime_mask', '0'),
+            ('hw:cpu_realtime_mask', 'a'),
             ('hw:cpu_realtime_mask', '^0,2-3,b'),
             ('hw:mem_page_size', 'largest'),
             ('hw:mem_page_size', '2kbits'),
diff --git a/nova/tests/unit/virt/test_hardware.py b/nova/tests/unit/virt/test_hardware.py
index 8888157db7ee..1c0da0860c9d 100644
--- a/nova/tests/unit/virt/test_hardware.py
+++ b/nova/tests/unit/virt/test_hardware.py
@@ -3818,6 +3818,42 @@ class CPURealtimeTestCase(test.NoDBTestCase):
         rt = hw.get_realtime_cpu_constraint(flavor, image)
         self.assertEqual(set([2]), rt)
 
+    def test_success_image_no_exclusion(self):
+        flavor = objects.Flavor(
+            vcpus=3, memory_mb=2048,
+            extra_specs={
+                'hw:cpu_realtime': 'true',
+            },
+        )
+        image = objects.ImageMeta.from_dict(
+            {"properties": {"hw_cpu_realtime_mask": "1-2"}})
+        rt = hw.get_realtime_cpu_constraint(flavor, image)
+        self.assertEqual(set([1, 2]), rt)
+
+    def test_success_image_leading_space(self):
+        flavor = objects.Flavor(
+            vcpus=3, memory_mb=2048,
+            extra_specs={
+                'hw:cpu_realtime': 'true',
+            },
+        )
+        image = objects.ImageMeta.from_dict(
+            {"properties": {"hw_cpu_realtime_mask": " ^1"}})
+        rt = hw.get_realtime_cpu_constraint(flavor, image)
+        self.assertEqual(set([0, 2]), rt)
+
+    def test_success_image_no_implicit_exclusion(self):
+        flavor = objects.Flavor(
+            vcpus=3, memory_mb=2048,
+            extra_specs={
+                'hw:cpu_realtime': 'true',
+            },
+        )
+        image = objects.ImageMeta.from_dict(
+            {"properties": {"hw_cpu_realtime_mask": "1-2,^1"}})
+        rt = hw.get_realtime_cpu_constraint(flavor, image)
+        self.assertEqual(set([2]), rt)
+
     def test_no_mask_configured(self):
         flavor = objects.Flavor(
             vcpus=3, memory_mb=2048,
diff --git a/nova/virt/hardware.py b/nova/virt/hardware.py
index b8af9553eb6e..64c9bf0bab62 100644
--- a/nova/virt/hardware.py
+++ b/nova/virt/hardware.py
@@ -1708,7 +1708,10 @@ def get_realtime_cpu_constraint(
 
     vcpus_set = set(range(flavor.vcpus))
     if mask:
-        vcpus_rt = parse_cpu_spec("0-%d,%s" % (flavor.vcpus - 1, mask))
+        if mask.strip().startswith('^'):
+            vcpus_rt = parse_cpu_spec("0-%d,%s" % (flavor.vcpus - 1, mask))
+        else:
+            vcpus_rt = parse_cpu_spec("%s" % (mask))
     else:
         vcpus_rt = set(range(flavor.vcpus))
 
diff --git a/releasenotes/notes/bug-1884231-16acf297d88b122e.yaml b/releasenotes/notes/bug-1884231-16acf297d88b122e.yaml
index 4f18a2fcb8b2..7ed64a0c1637 100644
--- a/releasenotes/notes/bug-1884231-16acf297d88b122e.yaml
+++ b/releasenotes/notes/bug-1884231-16acf297d88b122e.yaml
@@ -9,3 +9,8 @@ features:
     It is now possible to allocate all cores in an instance to realtime and
     omit the ``hw:cpu_realtime_mask`` extra spec. This requires specifying the
     ``hw:emulator_threads_policy`` extra spec.
+  - |
+    It is now possible to specify a mask in ``hw:cpu_realtime_mask`` without a
+    leading ``^``. When this is ommitted, the value will specify the cores that
+    should be included in the set of realtime cores, as opposed to those that
+    should be excluded.