qemu-options: New -compat to set policy for deprecated interfaces

New option -compat lets you configure what to do when deprecated
interfaces get used.  This is intended for testing users of the
management interfaces.  It is experimental.

-compat deprecated-input=<input-policy> configures what to do when
deprecated input is received.  Input policy can be "accept" (accept
silently), or "reject" (reject the request with an error).

-compat deprecated-output=<out-policy> configures what to do when
deprecated output is sent.  Output policy can be "accept" (pass on
unchanged), or "hide" (filter out the deprecated parts).

Default is "accept".  Policies other than "accept" are implemented
later in this series.

For now, -compat covers only syntactic aspects of QMP, i.e. stuff
tagged with feature 'deprecated'.  We may want to extend it to cover
semantic aspects, CLI, and experimental features.

Note that there is no good way for management application to detect
presence of -compat: it's not visible output of query-qmp-schema or
query-command-line-options.  Tolerable, because it's meant for
testing.  If running with -compat fails, skip the test.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210318155519.1224118-3-armbru@redhat.com>

7 files changed, 113 insertions, 0 deletions

diff --git a/include/qapi/compat-policy.h b/include/qapi/compat-policy.h
new file mode 100644
index 0000000..b8c6638
--- /dev/null
+++ b/include/qapi/compat-policy.h
@@ -0,0 +1,20 @@
+/*
+ * Policy for handling "funny" management interfaces
+ *
+ * Copyright (C) 2020 Red Hat, Inc.
+ *
+ * Authors:
+ *  Markus Armbruster <armbru@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or
+ * later.  See the COPYING file in the top-level directory.
+ */
+
+#ifndef QAPI_COMPAT_POLICY_H
+#define QAPI_COMPAT_POLICY_H
+
+#include "qapi/qapi-types-compat.h"
+
+extern CompatPolicy compat_policy;
+
+#endif
diff --git a/qapi/compat.json b/qapi/compat.json
new file mode 100644
index 0000000..fc24a58
--- /dev/null
+++ b/qapi/compat.json
@@ -0,0 +1,51 @@
+# -*- Mode: Python -*-
+
+##
+# = Compatibility policy
+##
+
+##
+# @CompatPolicyInput:
+#
+# Policy for handling "funny" input.
+#
+# @accept: Accept silently
+# @reject: Reject with an error
+#
+# Since: 6.0
+##
+{ 'enum': 'CompatPolicyInput',
+  'data': [ 'accept', 'reject' ] }
+
+##
+# @CompatPolicyOutput:
+#
+# Policy for handling "funny" output.
+#
+# @accept: Pass on unchanged
+# @hide: Filter out
+#
+# Since: 6.0
+##
+{ 'enum': 'CompatPolicyOutput',
+  'data': [ 'accept', 'hide' ] }
+
+##
+# @CompatPolicy:
+#
+# Policy for handling deprecated management interfaces.
+#
+# This is intended for testing users of the management interfaces.
+#
+# Limitation: covers only syntactic aspects of QMP, i.e. stuff tagged
+# with feature 'deprecated'.  We may want to extend it to cover
+# semantic aspects, CLI, and experimental features.
+#
+# @deprecated-input: how to handle deprecated input (default 'accept')
+# @deprecated-output: how to handle deprecated output (default 'accept')
+#
+# Since: 6.0
+##
+{ 'struct': 'CompatPolicy',
+  'data': { '*deprecated-input': 'CompatPolicyInput',
+            '*deprecated-output': 'CompatPolicyOutput' } }
diff --git a/qapi/meson.build b/qapi/meson.build
index d4424ae..0d20226 100644
--- a/qapi/meson.build
+++ b/qapi/meson.build
@@ -25,6 +25,7 @@ qapi_all_modules = [
   'block-export',
   'char',
   'common',
+  'compat',
   'control',
   'crypto',
   'dump',
diff --git a/qapi/qapi-schema.json b/qapi/qapi-schema.json
index 3441c9a..4912b97 100644
--- a/qapi/qapi-schema.json
+++ b/qapi/qapi-schema.json
@@ -79,6 +79,7 @@
 { 'include': 'migration.json' }
 { 'include': 'transaction.json' }
 { 'include': 'trace.json' }
+{ 'include': 'compat.json' }
 { 'include': 'control.json' }
 { 'include': 'introspect.json' }
 { 'include': 'qom.json' }
diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c
index 5e597c7..f6a65f5 100644
--- a/qapi/qmp-dispatch.c
+++ b/qapi/qmp-dispatch.c
@@ -14,6 +14,7 @@
 #include "qemu/osdep.h"
 
 #include "block/aio.h"
+#include "qapi/compat-policy.h"
 #include "qapi/error.h"
 #include "qapi/qmp/dispatch.h"
 #include "qapi/qmp/qdict.h"
@@ -23,6 +24,8 @@
 #include "qemu/coroutine.h"
 #include "qemu/main-loop.h"
 
+CompatPolicy compat_policy;
+
 static QDict *qmp_dispatch_check_obj(QDict *dict, bool allow_oob,
                                      Error **errp)
 {
diff --git a/qemu-options.hx b/qemu-options.hx
index 671b310..6b5a244 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3507,6 +3507,26 @@ DEFHEADING()
 
 DEFHEADING(Debug/Expert options:)
 
+DEF("compat", HAS_ARG, QEMU_OPTION_compat,
+    "-compat [deprecated-input=accept|reject][,deprecated-output=accept|hide]\n"
+    "                Policy for handling deprecated management interfaces\n",
+    QEMU_ARCH_ALL)
+SRST
+``-compat [deprecated-input=@var{input-policy}][,deprecated-output=@var{output-policy}]``
+    Set policy for handling deprecated management interfaces (experimental):
+
+    ``deprecated-input=accept`` (default)
+        Accept deprecated commands and arguments
+    ``deprecated-input=reject``
+        Reject deprecated commands and arguments
+    ``deprecated-output=accept`` (default)
+        Emit deprecated command results and events
+    ``deprecated-output=hide``
+        Suppress deprecated command results and events
+
+    Limitation: covers only syntactic aspects of QMP.
+ERST
+
 DEF("fw_cfg", HAS_ARG, QEMU_OPTION_fwcfg,
     "-fw_cfg [name=]<name>,file=<file>\n"
     "                add named fw_cfg entry with contents from file\n"
diff --git a/softmmu/vl.c b/softmmu/vl.c
index cba7ab3..4cbbdd8 100644
--- a/softmmu/vl.c
+++ b/softmmu/vl.c
@@ -29,6 +29,7 @@
 #include "exec/cpu-common.h"
 #include "hw/boards.h"
 #include "hw/qdev-properties.h"
+#include "qapi/compat-policy.h"
 #include "qapi/error.h"
 #include "qapi/qmp/qdict.h"
 #include "qapi/qmp/qjson.h"
@@ -114,6 +115,7 @@
 #include "sysemu/replay.h"
 #include "qapi/qapi-events-run-state.h"
 #include "qapi/qapi-visit-block-core.h"
+#include "qapi/qapi-visit-compat.h"
 #include "qapi/qapi-visit-ui.h"
 #include "qapi/qapi-commands-block-core.h"
 #include "qapi/qapi-commands-migration.h"
@@ -3458,6 +3460,21 @@ void qemu_init(int argc, char **argv, char **envp)
                 enable_mlock = qemu_opt_get_bool(opts, "mem-lock", false);
                 enable_cpu_pm = qemu_opt_get_bool(opts, "cpu-pm", false);
                 break;
+            case QEMU_OPTION_compat:
+                {
+                    CompatPolicy *opts;
+                    Visitor *v;
+
+                    v = qobject_input_visitor_new_str(optarg, NULL,
+                                                      &error_fatal);
+
+                    visit_type_CompatPolicy(v, NULL, &opts, &error_fatal);
+                    QAPI_CLONE_MEMBERS(CompatPolicy, &compat_policy, opts);
+
+                    qapi_free_CompatPolicy(opts);
+                    visit_free(v);
+                    break;
+                }
             case QEMU_OPTION_msg:
                 opts = qemu_opts_parse_noisily(qemu_find_opts("msg"), optarg,
                                                false);