Document caveats of 'VIR_DOMAIN_STATS_VM' group of statistics

The original patches adding the functionality neglected to add any form
of documentation for the stats fields returned for this group.

The stats are directly converted from qemu's 'query-stats(-schema)' QMP
command without any further interpretation. The 'query-stats-schema' has
the following disclaimer:

 Note: runtime-collected statistics and their names fall outside QEMU's usual
       deprecation policies.  QEMU will try to keep the set of available data
       stable, together with their names, but will not guarantee stability
       at all costs; the same is true of providers that source statistics
       externally, e.g. from Linux.  For example, if the same value is being
       tracked with different names on different architectures or by different
       providers, one of them might be renamed.  A statistic might go away if
       an algorithm is changed or some code is removed; changing a default
       might cause previously useful statistics to always report 0.  Such
       changes, however, are expected to be rare.

Since libvirt is not doing any form of conversion of the stats we can't
meaningfully document any of the returned fields. At the same time we
can't even meaningfully provide any form of API stability for the field
names.

Modify the documentation for the 'VIR_DOMAIN_STATS_VM' group both in the
API docs and in the virsh man page to reflect that and disclaim any form
of stability guarantees we provide normally.

Fixes: 8c9e3dae14
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>

docs/manpages/virsh.rst

@@ -2297,7 +2297,7 @@ domstats
 
    domstats [--raw] [--enforce] [--backing] [--nowait] [--state]
       [--cpu-total] [--balloon] [--vcpu] [--interface]
-      [--block] [--perf] [--iothread] [--memory] [--dirtyrate]
+      [--block] [--perf] [--iothread] [--memory] [--dirtyrate] [--vm]
       [[--list-active] [--list-inactive]
        [--list-persistent] [--list-transient] [--list-running]y
        [--list-paused] [--list-shutoff] [--list-other]] | [domain ...]
@@ -2317,7 +2317,7 @@ The individual statistics groups are selectable via specific flags. By
 default all supported statistics groups are returned. Supported
 statistics groups flags are: *--state*, *--cpu-total*, *--balloon*,
 *--vcpu*, *--interface*, *--block*, *--perf*, *--iothread*, *--memory*,
-*--dirtyrate*.
+*--dirtyrate*, *--vm*.
 
 Note that - depending on the hypervisor type and version or the domain state
 - not all of the following statistics may be returned.
@@ -2533,6 +2533,38 @@ not available for statistical purposes.
 * ``dirtyrate.vcpu.<num>.megabytes_per_second`` - the calculated memory dirty
   rate for a virtual cpu in MiB/s
 
+*--vm* returns:
+
+The *--vm* option enables reporting of hypervisor-specific statistics. Naming
+and meaning of the fields is entirely hypervisor dependent.
+
+The statistics in this group have the following naming scheme:
+
+ ``vm.$NAME.$TYPE``
+
+ ``$NAME``
+   name of the statistics field provided by the hypervisor
+
+ ``$TYPE``
+   Type of the value. The following types are returned:
+
+   ``cur``
+     current instant value
+   ``sum``
+     aggregate value
+   ``max``
+     peak value
+
+ The returned value may be either an unsigned long long or a boolean.
+
+ **WARNING**: The stats reported in this group are runtime-collected and
+ hypervisor originated, thus fall outside of the usual stable API
+ policies of libvirt.
+
+ Libvirt can't guarantee that the statistics reported from the outside
+ source will be present in further versions of the hypervisor, or that
+ naming or meaning will stay consistent. Changes to existing fields,
+ however, are expected to be rare.
 
 Selecting a specific statistics groups doesn't guarantee that the
 daemon supports the selected group of stats. Flag *--enforce*

src/libvirt-domain.c

@@ -12482,7 +12482,31 @@ virConnectGetDomainCapabilities(virConnectPtr conn,
  *                                                   unsigned long long.
  *
  * VIR_DOMAIN_STATS_VM:
- *     Return fd-based KVM statistics for the target VM
+ *     Return hypervisor-specific statistics. Note that the naming and meaning
+ *     of the fields is entirely hypervisor dependent.
+ *
+ *     The statistics in this group have the following naming scheme:
+ *
+ *     "vm.$NAME.$TYPE"
+ *
+ *       $NAME - name of the statistics field provided by the hypervisor
+ *
+ *       $TYPE - Type of the value. The following types are returned:
+ *          'cur' - current instant value
+ *          'sum' - aggregate value
+ *          'max' - peak value
+ *
+ *      The returned value may be either an unsigned long long or a boolean.
+ *
+ *     WARNING:
+ *      The stats reported in this group are runtime-collected and
+ *      hypervisor originated, thus fall outside of the usual stable API
+ *      policies of libvirt.
+ *
+ *      Libvirt can't guarantee that the statistics reported from the outside
+ *      source will be present in further versions of the hypervisor, or that
+ *      naming or meaning will stay consistent. Changes to existing fields,
+ *      however, are expected to be rare.
  *
  * Note that entire stats groups or individual stat fields may be missing from
  * the output in case they are not supported by the given hypervisor, are not

tools/virsh-domain-monitor.c

@@ -2067,7 +2067,7 @@ static const vshCmdOptDef opts_domstats[] = {
     },
     {.name = "vm",
      .type = VSH_OT_BOOL,
-     .help = N_("report fd-based VM statistics by KVM"),
+     .help = N_("report hypervisor-specific statistics"),
     },
     {.name = "list-active",
      .type = VSH_OT_BOOL,