--- a/python/mach/mach/registrar.py
+++ b/python/mach/mach/registrar.py
@@ -111,16 +111,20 @@ class MachRegistrar(object):
 
         if handler.parser:
             parser = handler.parser
 
             # save and restore existing defaults so **kwargs don't persist across
             # subsequent invocations of Registrar.dispatch()
             old_defaults = parser._defaults.copy()
             parser.set_defaults(**kwargs)
-            kwargs, _ = parser.parse_known_args(argv or [])
+            kwargs, unknown = parser.parse_known_args(argv or [])
             kwargs = vars(kwargs)
             parser._defaults = old_defaults
 
+            if unknown:
+                print("warning: unknown arguments {}".format(
+                    ', '.join(["'{}'".format(arg) for arg in unknown])))
+
         return self._run_command_handler(handler, context=context, **kwargs)
 
 
 Registrar = MachRegistrar()

--- a/testing/mach_commands.py
+++ b/testing/mach_commands.py
@@ -5,16 +5,17 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
 import json
 import os
 import sys
 import tempfile
 import subprocess
 import shutil
+from argparse import REMAINDER
 from collections import defaultdict
 
 from mach.decorators import (
     CommandArgument,
     CommandProvider,
     Command,
 )
 
@@ -192,17 +193,20 @@ The following test suites and aliases ar
 ''' % ', '.join(sorted(TEST_SUITES))
 TEST_HELP = TEST_HELP.strip()
 
 
 @CommandProvider
 class Test(MachCommandBase):
     @Command('test', category='testing', description='Run tests (detects the kind of test and runs it).')
     @CommandArgument('what', default=None, nargs='*', help=TEST_HELP)
-    def test(self, what):
+    @CommandArgument('extra_args', default=None, nargs=REMAINDER,
+        help="Extra arguments to pass to the underlying test command(s). If an underlying "
+             "command doesn't recognize the argument, it will be ignored for that test harness.")
+    def test(self, what, extra_args):
         """Run tests from names or paths.
 
         mach test accepts arguments specifying which tests to run. Each argument
         can be:
 
         * The path to a test file
         * A directory containing tests
         * A test suite name
@@ -295,17 +299,17 @@ class Test(MachCommandBase):
 
         status = None
         for suite_name in run_suites:
             suite = TEST_SUITES[suite_name]
 
             if 'mach_command' in suite:
                 res = self._mach_context.commands.dispatch(
                     suite['mach_command'], self._mach_context,
-                    **suite['kwargs'])
+                    argv=extra_args, **suite['kwargs'])
                 if res:
                     status = res
 
         buckets = {}
         for test in run_tests:
             key = (test['flavor'], test['subsuite'])
             buckets.setdefault(key, []).append(test)
 
@@ -321,17 +325,17 @@ class Test(MachCommandBase):
                 status = 1
                 continue
 
             kwargs = dict(m['kwargs'])
             kwargs['subsuite'] = subsuite
 
             res = self._mach_context.commands.dispatch(
                     m['mach_command'], self._mach_context,
-                    test_objects=tests, **kwargs)
+                    argv=extra_args, test_objects=tests, **kwargs)
             if res:
                 status = res
 
         return status
 
 
 @CommandProvider
 class MachCommands(MachCommandBase):