[v2,05/22] block: Add idle_time_ns to BlockDeviceStats

diff --git a/block/accounting.c b/block/accounting.c
index 6f4c0f1..d427fa8 100644
--- a/block/accounting.c
+++ b/block/accounting.c
@@ -40,12 +40,15 @@  void block_acct_start(BlockAcctStats *stats, BlockAcctCookie *cookie,
 
 void block_acct_done(BlockAcctStats *stats, BlockAcctCookie *cookie)
 {
+    int64_t time_ns = qemu_clock_get_ns(clock_type);
+    int64_t latency_ns = time_ns - cookie->start_time_ns;
+
     assert(cookie->type < BLOCK_MAX_IOTYPE);
 
     stats->nr_bytes[cookie->type] += cookie->bytes;
     stats->nr_ops[cookie->type]++;
-    stats->total_time_ns[cookie->type] +=
-        qemu_clock_get_ns(clock_type) - cookie->start_time_ns;
+    stats->total_time_ns[cookie->type] += latency_ns;
+    stats->last_access_time_ns = time_ns;
 }
 
 
@@ -55,3 +58,8 @@  void block_acct_merge_done(BlockAcctStats *stats, enum BlockAcctType type,
     assert(type < BLOCK_MAX_IOTYPE);
     stats->merged[type] += num_requests;
 }
+
+int64_t block_acct_idle_time_ns(BlockAcctStats *stats)
+{
+    return qemu_clock_get_ns(clock_type) - stats->last_access_time_ns;
+}
diff --git a/block/qapi.c b/block/qapi.c
index e936ba7..66f2604 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -357,6 +357,11 @@  static BlockStats *bdrv_query_stats(const BlockDriverState *bs,
         s->stats->wr_total_time_ns = stats->total_time_ns[BLOCK_ACCT_WRITE];
         s->stats->rd_total_time_ns = stats->total_time_ns[BLOCK_ACCT_READ];
         s->stats->flush_total_time_ns = stats->total_time_ns[BLOCK_ACCT_FLUSH];
+
+        s->stats->has_idle_time_ns = stats->last_access_time_ns > 0;
+        if (s->stats->has_idle_time_ns) {
+            s->stats->idle_time_ns = block_acct_idle_time_ns(stats);
+        }
     }
 
     s->stats->wr_highest_offset = bs->wr_highest_offset;
diff --git a/hmp.c b/hmp.c
index 6e2bbc8..289c240 100644
--- a/hmp.c
+++ b/hmp.c
@@ -511,6 +511,7 @@  void hmp_info_blockstats(Monitor *mon, const QDict *qdict)
                        " flush_total_time_ns=%" PRId64
                        " rd_merged=%" PRId64
                        " wr_merged=%" PRId64
+                       " idle_time_ns=%" PRId64
                        "\n",
                        stats->value->stats->rd_bytes,
                        stats->value->stats->wr_bytes,
@@ -521,7 +522,8 @@  void hmp_info_blockstats(Monitor *mon, const QDict *qdict)
                        stats->value->stats->rd_total_time_ns,
                        stats->value->stats->flush_total_time_ns,
                        stats->value->stats->rd_merged,
-                       stats->value->stats->wr_merged);
+                       stats->value->stats->wr_merged,
+                       stats->value->stats->idle_time_ns);
     }
 
     qapi_free_BlockStatsList(stats_list);
diff --git a/include/block/accounting.h b/include/block/accounting.h
index 66637cd..4b2b999 100644
--- a/include/block/accounting.h
+++ b/include/block/accounting.h
@@ -40,6 +40,7 @@  typedef struct BlockAcctStats {
     uint64_t nr_ops[BLOCK_MAX_IOTYPE];
     uint64_t total_time_ns[BLOCK_MAX_IOTYPE];
     uint64_t merged[BLOCK_MAX_IOTYPE];
+    int64_t last_access_time_ns;
 } BlockAcctStats;
 
 typedef struct BlockAcctCookie {
@@ -53,5 +54,6 @@  void block_acct_start(BlockAcctStats *stats, BlockAcctCookie *cookie,
 void block_acct_done(BlockAcctStats *stats, BlockAcctCookie *cookie);
 void block_acct_merge_done(BlockAcctStats *stats, enum BlockAcctType type,
                            int num_requests);
+int64_t block_acct_idle_time_ns(BlockAcctStats *stats);
 
 #endif
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 5f12af7..a529e2f 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -448,6 +448,10 @@ 
 # @wr_merged: Number of write requests that have been merged into another
 #             request (Since 2.3).
 #
+# @idle_time_ns: #optional Time since the last I/O operation, in
+#                miliseconds. If the field is absent it means that
+#                there haven't been any operations yet (Since 2.5).
+#
 # Since: 0.14.0
 ##
 { 'struct': 'BlockDeviceStats',
@@ -455,7 +459,7 @@ 
            'wr_operations': 'int', 'flush_operations': 'int',
            'flush_total_time_ns': 'int', 'wr_total_time_ns': 'int',
            'rd_total_time_ns': 'int', 'wr_highest_offset': 'int',
-           'rd_merged': 'int', 'wr_merged': 'int' } }
+           'rd_merged': 'int', 'wr_merged': 'int', '*idle_time_ns': 'int' } }
 
 ##
 # @BlockStats:
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 4f03d11..13c1bdc 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -2511,6 +2511,10 @@  Each json-object contain the following:
                    another request (json-int)
     - "wr_merged": number of write requests that have been merged into
                    another request (json-int)
+    - "idle_time_ns": time since the last I/O operation, in
+                      miliseconds. If the field is absent it means
+                      that there haven't been any operations yet
+                      (json-int, optional)
 - "parent": Contains recursively the statistics of the underlying
             protocol (e.g. the host file for a qcow2 image). If there is
             no underlying protocol, this field is omitted
@@ -2535,7 +2539,8 @@  Example:
                   "flush_total_times_ns":49653
                   "flush_operations":61,
                   "rd_merged":0,
-                  "wr_merged":0
+                  "wr_merged":0,
+                  "idle_time_ns":2953431879
                }
             },
             "stats":{
@@ -2549,7 +2554,8 @@  Example:
                "rd_total_times_ns":3465673657
                "flush_total_times_ns":49653,
                "rd_merged":0,
-               "wr_merged":0
+               "wr_merged":0,
+               "idle_time_ns":2953431879
             }
          },
          {