[PATCH v2 2/8] qapi: add 'warn' option to CompatPolicyInput

["Daniel P. Berrangé" <berrange@redhat.com>]

This prints a warning on stderr every time a policy violation
is encountered, but accepts the input. Warning every time is
known to be annoying and sub-optimal, but the normal trick of
using warn_report_once() isn't possible in this scenario.

As a result 'warn' is not set as the default policy at this
time, despite other manually placed deprecation warnings being
emitted by default.

Future work will consider ways to reduce the warnings to be
once only, which would unlock the ability to use 'warn' by
default.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 qapi/compat.json |  7 ++++++-
 qapi/qapi-util.c |  4 ++++
 qemu-options.hx  | 14 ++++++++++----
 3 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/qapi/compat.json b/qapi/compat.json
index 0f90ea1f524..60591468ec0 100644
--- a/qapi/compat.json
+++ b/qapi/compat.json
@@ -17,10 +17,15 @@
 #
 # @crash: abort() the process
 #
+# @warn: Accept with warnings printed to stderr (Since: 11.2)
+#
+# .. note:: at this time, warnings will be printed on every policy
+#           check failure rather than once only.
+#
 # Since: 6.0
 ##
 { 'enum': 'CompatPolicyInput',
-  'data': [ 'accept', 'reject', 'crash' ] }
+  'data': [ 'accept', 'reject', 'crash', 'warn' ] }
 
 ##
 # @CompatPolicyOutput:
diff --git a/qapi/qapi-util.c b/qapi/qapi-util.c
index 93898f57a05..fe0a6a57c85 100644
--- a/qapi/qapi-util.c
+++ b/qapi/qapi-util.c
@@ -28,6 +28,10 @@ static bool compat_policy_input_ok1(CompatPolicyInput policy,
     switch (policy) {
     case COMPAT_POLICY_INPUT_ACCEPT:
         return true;
+    case COMPAT_POLICY_INPUT_WARN:
+        warn_report("%s %s %s accepted by policy",
+                    adjective, kind, name);
+        return true;
     case COMPAT_POLICY_INPUT_REJECT:
         error_set(errp, error_class, "%s %s %s disabled by policy",
                   adjective, kind, name);
diff --git a/qemu-options.hx b/qemu-options.hx
index f72f30e246d..854341e7a07 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4693,9 +4693,9 @@ DEFHEADING()
 DEFHEADING(Debug/Expert options:)
 
 DEF("compat", HAS_ARG, QEMU_OPTION_compat,
-    "-compat [deprecated-input=accept|reject|crash][,deprecated-output=accept|hide]\n"
+    "-compat [deprecated-input=accept|reject|crash|warn][,deprecated-output=accept|hide]\n"
     "                Policy for handling deprecated management interfaces\n"
-    "-compat [unstable-input=accept|reject|crash][,unstable-output=accept|hide]\n"
+    "-compat [unstable-input=accept|reject|crash|warn][,unstable-output=accept|hide]\n"
     "                Policy for handling unstable management interfaces\n",
     QEMU_ARCH_ALL)
 SRST
@@ -4703,7 +4703,10 @@ SRST
     Set policy for handling deprecated management interfaces (experimental):
 
     ``deprecated-input=accept`` (default)
-        Accept deprecated commands and arguments
+        Accept deprecated commands and arguments silently
+    ``deprecated-input=warn``
+        Accept deprecated commands and arguments with repeated warnings
+        on stderr
     ``deprecated-input=reject``
         Reject deprecated commands and arguments
     ``deprecated-input=crash``
@@ -4719,7 +4722,10 @@ SRST
     Set policy for handling unstable management interfaces (experimental):
 
     ``unstable-input=accept`` (default)
-        Accept unstable commands and arguments
+        Accept unstable commands and arguments silently
+    ``unstable-input=warn``
+        Accept unstable commands and arguments with repeated warnings
+        on stderr
     ``unstable-input=reject``
         Reject unstable commands and arguments
     ``unstable-input=crash``
-- 
2.56.0