qmp: add filtering of statistics by name

Allow retrieving only a subset of statistics.  This can be useful
for example in order to plot a subset of the statistics many times
a second: KVM publishes ~40 statistics for each vCPU on x86; retrieving
and serializing all of them would be useless.

Another use will be in HMP in the following patch; implementing the
filter in the backend is easy enough that it was deemed okay to make
this a public interface.

Example:

{ "execute": "query-stats",
  "arguments": {
    "target": "vcpu",
    "vcpus": [ "/machine/unattached/device[2]",
               "/machine/unattached/device[4]" ],
    "providers": [
      { "provider": "kvm",
        "names": [ "l1d_flush", "exits" ] } } }

{ "return": {
    "vcpus": [
      { "path": "/machine/unattached/device[2]"
        "providers": [
          { "provider": "kvm",
            "stats": [ { "name": "l1d_flush", "value": 41213 },
                       { "name": "exits", "value": 74291 } ] } ] },
      { "path": "/machine/unattached/device[4]"
        "providers": [
          { "provider": "kvm",
            "stats": [ { "name": "l1d_flush", "value": 16132 },
                       { "name": "exits", "value": 57922 } ] } ] } ] } }

Extracted from a patch by Mark Kanda.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/monitor/qmp-cmds.c b/monitor/qmp-cmds.c
index e49ab345d7..7314cd813d 100644
--- a/monitor/qmp-cmds.c
+++ b/monitor/qmp-cmds.c
@@ -472,12 +472,17 @@ static bool invoke_stats_cb(StatsCallbacks *entry,
                             Error **errp)
 {
     strList *targets = NULL;
+    strList *names = NULL;
     ERRP_GUARD();
 
     if (request) {
         if (request->provider != entry->provider) {
             return true;
         }
+        if (request->has_names && !request->names) {
+            return true;
+        }
+        names = request->has_names ? request->names : NULL;
     }
 
     switch (filter->target) {
@@ -496,7 +501,7 @@ static bool invoke_stats_cb(StatsCallbacks *entry,
         abort();
     }
 
-    entry->stats_cb(stats_results, filter->target, targets, errp);
+    entry->stats_cb(stats_results, filter->target, names, targets, errp);
     if (*errp) {
         qapi_free_StatsResultList(*stats_results);
         *stats_results = NULL;