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>

accel/kvm/kvm-all.c

@@ -2312,7 +2312,7 @@ bool kvm_dirty_ring_enabled(void)
 }
 
 static void query_stats_cb(StatsResultList **result, StatsTarget target,
-                           strList *targets, Error **errp);
+                           strList *names, strList *targets, Error **errp);
 static void query_stats_schemas_cb(StatsSchemaList **result, Error **errp);
 
 static int kvm_init(MachineState *ms)
@@ -3713,6 +3713,7 @@ typedef struct StatsArgs {
         StatsResultList **stats;
         StatsSchemaList **schema;
     } result;
+    strList *names;
     Error **errp;
 } StatsArgs;
 
@@ -3916,7 +3917,7 @@ static StatsDescriptors *find_stats_descriptors(StatsTarget target, int stats_fd
 }
 
 static void query_stats(StatsResultList **result, StatsTarget target,
-                        int stats_fd, Error **errp)
+                        strList *names, int stats_fd, Error **errp)
 {
     struct kvm_stats_desc *kvm_stats_desc;
     struct kvm_stats_header *kvm_stats_header;
@@ -3958,6 +3959,9 @@ static void query_stats(StatsResultList **result, StatsTarget target,
 
         /* Add entry to the list */
         stats = (void *)stats_data + pdesc->offset;
+        if (!apply_str_list_filter(pdesc->name, names)) {
+            continue;
+        }
         stats_list = add_kvmstat_entry(pdesc, stats, stats_list, errp);
     }
 
@@ -4019,8 +4023,8 @@ static void query_stats_vcpu(CPUState *cpu, run_on_cpu_data data)
         error_propagate(kvm_stats_args->errp, local_err);
         return;
     }
-    query_stats(kvm_stats_args->result.stats, STATS_TARGET_VCPU, stats_fd,
-                kvm_stats_args->errp);
+    query_stats(kvm_stats_args->result.stats, STATS_TARGET_VCPU,
+                kvm_stats_args->names, stats_fd, kvm_stats_args->errp);
     close(stats_fd);
 }
 
@@ -4041,7 +4045,7 @@ static void query_stats_schema_vcpu(CPUState *cpu, run_on_cpu_data data)
 }
 
 static void query_stats_cb(StatsResultList **result, StatsTarget target,
-                           strList *targets, Error **errp)
+                           strList *names, strList *targets, Error **errp)
 {
     KVMState *s = kvm_state;
     CPUState *cpu;
@@ -4055,7 +4059,7 @@ static void query_stats_cb(StatsResultList **result, StatsTarget target,
             error_setg_errno(errp, errno, "KVM stats: ioctl failed");
             return;
         }
-        query_stats(result, target, stats_fd, errp);
+        query_stats(result, target, names, stats_fd, errp);
         close(stats_fd);
         break;
     }
@@ -4063,6 +4067,7 @@ static void query_stats_cb(StatsResultList **result, StatsTarget target,
     {
         StatsArgs stats_args;
         stats_args.result.stats = result;
+        stats_args.names = names;
         stats_args.errp = errp;
         CPU_FOREACH(cpu) {
             if (!apply_str_list_filter(cpu->parent_obj.canonical_path, targets)) {

include/monitor/stats.h

@@ -11,7 +11,7 @@
 #include "qapi/qapi-types-stats.h"
 
 typedef void StatRetrieveFunc(StatsResultList **result, StatsTarget target,
-                              strList *targets, Error **errp);
+                              strList *names, strList *targets, Error **errp);
 typedef void SchemaRetrieveFunc(StatsSchemaList **result, Error **errp);
 
 /*

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;

qapi/stats.json

@@ -75,11 +75,14 @@
 # Indicates a set of statistics that should be returned by query-stats.
 #
 # @provider: provider for which to return statistics.
+
+# @names: statistics to be returned (all if omitted).
 #
 # Since: 7.1
 ##
 { 'struct': 'StatsRequest',
-  'data': { 'provider': 'StatsProvider' } }
+  'data': { 'provider': 'StatsProvider',
+            '*names': [ 'str' ] } }
 
 ##
 # @StatsVCPUFilter:
@@ -99,6 +102,7 @@
 # that target:
 # - which vCPUs to request statistics for
 # - which providers to request statistics from
+# - which named values to return within each provider
 #
 # Since: 7.1
 ##