[Qemu-devel] [PATCH] qmp: add query-qemu-capabilities

[Stefan Hajnoczi <stefanha@redhat.com>]

QMP clients can usually detect the presence of features via schema
introspection.  There are rare features that do not involve schema
changes and are therefore impossible to detect with schema
introspection.

This patch adds the query-qemu-capabilities command.  It returns a list
of capabilities that this QEMU supports.

The decision to make this a command rather than something statically
defined in the schema is intentional.  It allows QEMU to decide which
capabilities are available at runtime, if necessary.

This new interface is necessary so that QMP clients can discover that
migrating disk image files is safe with cache.direct=off on Linux.
There is no other way to detect whether or not QEMU supports this.

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 qapi/misc.json | 42 ++++++++++++++++++++++++++++++++++++++++++
 qmp.c          | 18 ++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/qapi/misc.json b/qapi/misc.json
index 8b3ca4fdd3..9cf24919a3 100644
--- a/qapi/misc.json
+++ b/qapi/misc.json
@@ -3051,3 +3051,45 @@
   'data': 'NumaOptions',
   'allow-preconfig': true
 }
+
+##
+# @QemuCapability:
+#
+# Capabilities that are not otherwise introspectable.
+#
+# @migrate-with-cache-direct-off-on-linux:
+#  It is safe to migrate disk image files with cache.direct=off on Linux.
+#  Previously only cache.direct=on was supported for live migration.
+#
+# Since: 4.0
+##
+{ 'enum': 'QemuCapability',
+  'data': [
+    'migrate-with-cache-direct-off-on-linux'
+  ]
+}
+
+##
+# @QemuCapabilities:
+#
+# Capability information.
+#
+# @capabilities: supported capabilities
+#
+# Since: 4.0
+##
+{ 'struct': 'QemuCapabilities',
+  'data': { 'capabilities': ['QemuCapability'] } }
+
+##
+# @query-qemu-capabilities:
+#
+# Return the capabilities supported by this QEMU.  Most features can be
+# detected via schema introspection but some are not observable from the
+# schema.  This command offers a way to check for the presence of such
+# features.
+#
+# Since: 4.0
+##
+{ 'command': 'query-qemu-capabilities',
+  'returns': 'QemuCapabilities' }
diff --git a/qmp.c b/qmp.c
index b92d62cd5f..91a1228be7 100644
--- a/qmp.c
+++ b/qmp.c
@@ -717,3 +717,21 @@ MemoryInfo *qmp_query_memory_size_summary(Error **errp)
 
     return mem_info;
 }
+
+QemuCapabilities *qmp_query_qemu_capabilities(Error **errp)
+{
+    QemuCapabilities *caps = g_new0(QemuCapabilities, 1);
+    QemuCapabilityList **prev = &caps->capabilities;
+    QemuCapability cap_val;
+
+    /* Add all QemuCapability enum values defined in the schema */
+    for (cap_val = 0; cap_val < QEMU_CAPABILITY__MAX; cap_val++) {
+        QemuCapabilityList *cap = g_new0(QemuCapabilityList, 1);
+
+        cap->value = cap_val;
+        *prev = cap;
+        prev = &cap->next;
+    }
+
+    return caps;
+}
-- 
2.20.1