qmp: add filtering of statistics by target vCPU

Introduce a simple filtering of statistics, that allows to retrieve
statistics for a subset of the guest vCPUs.  This will be used for
example by the HMP monitor, in order to retrieve the statistics
for the currently selected CPU.

Example:
{ "execute": "query-stats",
  "arguments": {
    "target": "vcpu",
    "vcpus": [ "/machine/unattached/device[2]",
               "/machine/unattached/device[4]" ] } }

Extracted from a patch by Mark Kanda.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -2311,7 +2311,8 @@ bool kvm_dirty_ring_enabled(void)
     return kvm_state->kvm_dirty_ring_size ? true : false;
 }
 
-static void query_stats_cb(StatsResultList **result, StatsTarget target, Error **errp);
+static void query_stats_cb(StatsResultList **result, StatsTarget target,
+                           strList *targets, Error **errp);
 static void query_stats_schemas_cb(StatsSchemaList **result, Error **errp);
 
 static int kvm_init(MachineState *ms)
@@ -4038,7 +4039,8 @@ static void query_stats_schema_vcpu(CPUState *cpu, run_on_cpu_data data)
     close(stats_fd);
 }
 
-static void query_stats_cb(StatsResultList **result, StatsTarget target, Error **errp)
+static void query_stats_cb(StatsResultList **result, StatsTarget target,
+                           strList *targets, Error **errp)
 {
     KVMState *s = kvm_state;
     CPUState *cpu;
@@ -4062,6 +4064,9 @@ static void query_stats_cb(StatsResultList **result, StatsTarget target, Error *
         stats_args.result.stats = result;
         stats_args.errp = errp;
         CPU_FOREACH(cpu) {
+            if (!apply_str_list_filter(cpu->parent_obj.canonical_path, targets)) {
+                continue;
+            }
             run_on_cpu(cpu, query_stats_vcpu, RUN_ON_CPU_HOST_PTR(&stats_args));
         }
         break;

include/monitor/stats.h

@@ -11,7 +11,7 @@
 #include "qapi/qapi-types-stats.h"
 
 typedef void StatRetrieveFunc(StatsResultList **result, StatsTarget target,
-                              Error **errp);
+                              strList *targets, Error **errp);
 typedef void SchemaRetrieveFunc(StatsSchemaList **result, Error **errp);
 
 /*
@@ -31,4 +31,13 @@ void add_stats_entry(StatsResultList **, StatsProvider, const char *id,
 void add_stats_schema(StatsSchemaList **, StatsProvider, StatsTarget,
                       StatsSchemaValueList *);
 
+/*
+ * True if a string matches the filter passed to the stats_fn callabck,
+ * false otherwise.
+ *
+ * Note that an empty list means no filtering, i.e. all strings will
+ * return true.
+ */
+bool apply_str_list_filter(const char *string, strList *list);
+
 #endif /* STATS_H */

monitor/qmp-cmds.c

@@ -468,9 +468,26 @@ static bool invoke_stats_cb(StatsCallbacks *entry,
                             StatsFilter *filter,
                             Error **errp)
 {
+    strList *targets = NULL;
     ERRP_GUARD();
 
-    entry->stats_cb(stats_results, filter->target, errp);
+    switch (filter->target) {
+    case STATS_TARGET_VM:
+        break;
+    case STATS_TARGET_VCPU:
+        if (filter->u.vcpu.has_vcpus) {
+            if (!filter->u.vcpu.vcpus) {
+                /* No targets allowed?  Return no statistics.  */
+                return true;
+            }
+            targets = filter->u.vcpu.vcpus;
+        }
+        break;
+    default:
+        abort();
+    }
+
+    entry->stats_cb(stats_results, filter->target, targets, errp);
     if (*errp) {
         qapi_free_StatsResultList(*stats_results);
         *stats_results = NULL;
@@ -536,3 +553,18 @@ void add_stats_schema(StatsSchemaList **schema_results,
     entry->stats = stats_list;
     QAPI_LIST_PREPEND(*schema_results, entry);
 }
+
+bool apply_str_list_filter(const char *string, strList *list)
+{
+    strList *str_list = NULL;
+
+    if (!list) {
+        return true;
+    }
+    for (str_list = list; str_list; str_list = str_list->next) {
+        if (g_str_equal(string, str_list->value)) {
+            return true;
+        }
+    }
+    return false;
+}

qapi/stats.json

@@ -69,16 +69,30 @@
 { 'enum': 'StatsTarget',
   'data': [ 'vm', 'vcpu' ] }
 
+##
+# @StatsVCPUFilter:
+#
+# @vcpus: list of QOM paths for the desired vCPU objects.
+#
+# Since: 7.1
+##
+{ 'struct': 'StatsVCPUFilter',
+  'data': { '*vcpus': [ 'str' ] } }
+
 ##
 # @StatsFilter:
 #
 # The arguments to the query-stats command; specifies a target for which to
-# request statistics.
+# request statistics and optionally the required subset of information for
+# that target:
+# - which vCPUs to request statistics for
 #
 # Since: 7.1
 ##
-{ 'struct': 'StatsFilter',
-  'data': { 'target': 'StatsTarget' } }
+{ 'union': 'StatsFilter',
+        'base': { 'target': 'StatsTarget' },
+  'discriminator': 'target',
+  'data': { 'vcpu': 'StatsVCPUFilter' } }
 
 ##
 # @StatsValue: