Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2019-06-12' into staging

QAPI patches for 2019-06-12

# gpg: Signature made Wed 12 Jun 2019 17:44:50 BST
# gpg:                using RSA key 354BC8B3D7EB2A6B68674E5F3870B400EB918653
# gpg:                issuer "armbru@redhat.com"
# gpg: Good signature from "Markus Armbruster <armbru@redhat.com>" [full]
# gpg:                 aka "Markus Armbruster <armbru@pond.sub.org>" [full]
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* remotes/armbru/tags/pull-qapi-2019-06-12:
  qapi: Simplify how QAPIDoc implements its state machine
  file-posix: Add dynamic-auto-read-only QAPI feature
  qapi: Allow documentation for features
  qapi: Disentangle QAPIDoc code
  tests/qapi-schema: Error case tests for features in structs
  tests/qapi-schema: Test for good feature lists in structs
  qapi: Add feature flags to struct types
  block/gluster: update .help of BLOCK_OPT_PREALLOC option
  block/file-posix: update .help of BLOCK_OPT_PREALLOC option
  qapi/block-core: update documentation of preallocation parameter
  qdev: Delete unused LostTickPolicy "merge"

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 38 insertions, 0 deletions

diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
index b517b0cfbf..e8ec8ac1de 100644
--- a/docs/devel/qapi-code-gen.txt
+++ b/docs/devel/qapi-code-gen.txt
@@ -719,6 +719,34 @@ any non-empty complex type (struct, union, or alternate), and a
 pointer to that QAPI type is passed as a single argument.
 
 
+=== Features ===
+
+Sometimes, the behaviour of QEMU changes compatibly, but without a
+change in the QMP syntax (usually by allowing values or operations that
+previously resulted in an error). QMP clients may still need to know
+whether the extension is available.
+
+For this purpose, a list of features can be specified for a struct type.
+This is exposed to the client as a list of string, where each string
+signals that this build of QEMU shows a certain behaviour.
+
+In the schema, features can be specified as simple strings, for example:
+
+{ 'struct': 'TestType',
+  'data': { 'number': 'int' },
+  'features': [ 'allow-negative-numbers' ] }
+
+Another option is to specify features as dictionaries, where the key
+'name' specifies the feature string to be exposed to clients:
+
+{ 'struct': 'TestType',
+  'data': { 'number': 'int' },
+  'features': [ { 'name': 'allow-negative-numbers' } ] }
+
+This expanded form is necessary if you want to make the feature
+conditional (see below in "Configuring the schema").
+
+
 === Downstream extensions ===
 
 QAPI schema names that are externally visible, say in the Client JSON
@@ -771,6 +799,16 @@ Example: a conditional 'bar' enum member.
   [ 'foo',
     { 'name' : 'bar', 'if': 'defined(IFCOND)' } ] }
 
+Similarly, features can be specified as a dictionary with a 'name' and
+an 'if' key.
+
+Example: a conditional 'allow-negative-numbers' feature
+
+{ 'struct': 'TestType',
+  'data': { 'number': 'int' },
+  'features': [ { 'name': 'allow-negative-numbers',
+                  'if' 'defined(IFCOND)' } ] }
+
 Please note that you are responsible to ensure that the C code will
 compile with an arbitrary combination of conditions, since the
 generators are unable to check it at this point.