qemu-img info: Optionally show block limits

Add a new --limits option to 'qemu-img info' that displays the block
limits for the image and all of its children, making the information
more accessible for human users than in QMP. This option is not enabled
by default because it can be a lot of output that isn't usually relevant
if you're not specifically trying to diagnose some I/O problem.

This makes the same information automatically also available in HMP
'info block -v'.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20251024123041.51254-4-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qapi.c

@@ -417,6 +417,7 @@ fail:
  */
 void bdrv_query_block_graph_info(BlockDriverState *bs,
                                  BlockGraphInfo **p_info,
+                                 bool limits,
                                  Error **errp)
 {
     ERRP_GUARD();
@@ -425,7 +426,7 @@ void bdrv_query_block_graph_info(BlockDriverState *bs,
     BdrvChild *c;
 
     info = g_new0(BlockGraphInfo, 1);
-    bdrv_do_query_node_info(bs, qapi_BlockGraphInfo_base(info), false, errp);
+    bdrv_do_query_node_info(bs, qapi_BlockGraphInfo_base(info), limits, errp);
     if (*errp) {
         goto fail;
     }
@@ -439,7 +440,7 @@ void bdrv_query_block_graph_info(BlockDriverState *bs,
         QAPI_LIST_APPEND(children_list_tail, c_info);
 
         c_info->name = g_strdup(c->name);
-        bdrv_query_block_graph_info(c->bs, &c_info->info, errp);
+        bdrv_query_block_graph_info(c->bs, &c_info->info, limits, errp);
         if (*errp) {
             goto fail;
         }
@@ -936,6 +937,29 @@ void bdrv_image_info_specific_dump(ImageInfoSpecific *info_spec,
     visit_free(v);
 }
 
+/**
+ * Dumps the given BlockLimitsInfo object in a human-readable form,
+ * prepending an optional prefix if the dump is not empty.
+ */
+static void bdrv_image_info_limits_dump(BlockLimitsInfo *limits,
+                                        const char *prefix,
+                                        int indentation)
+{
+    QObject *obj;
+    Visitor *v = qobject_output_visitor_new(&obj);
+
+    visit_type_BlockLimitsInfo(v, NULL, &limits, &error_abort);
+    visit_complete(v, &obj);
+    if (!qobject_is_empty_dump(obj)) {
+        if (prefix) {
+            qemu_printf("%*s%s", indentation * 4, "", prefix);
+        }
+        dump_qobject(indentation + 1, obj);
+    }
+    qobject_unref(obj);
+    visit_free(v);
+}
+
 /**
  * Print the given @info object in human-readable form.  Every field is indented
  * using the given @indentation (four spaces per indentation level).
@@ -1011,6 +1035,12 @@ void bdrv_node_info_dump(BlockNodeInfo *info, int indentation, bool protocol)
         }
     }
 
+    if (info->limits) {
+        bdrv_image_info_limits_dump(info->limits,
+                                    "Block limits:\n",
+                                    indentation);
+    }
+
     if (info->has_snapshots) {
         SnapshotInfoList *elem;
 

docs/tools/qemu-img.rst

@@ -503,7 +503,7 @@ Command description:
 
   The size syntax is similar to :manpage:`dd(1)`'s size syntax.
 
-.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
+.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-U] FILENAME
 
   Give information about the disk image *FILENAME*. Use it in
   particular to know the size reserved on disk which can be different
@@ -571,6 +571,10 @@ Command description:
     ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2``
     for qcow2 images).
 
+  *Block limits*
+    The block limits for I/O that QEMU detected for the image.
+    This information is only shown if the ``--limits`` option was specified.
+
 .. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
 
   Dump the metadata of image *FILENAME* and its backing file chain.

include/block/qapi.h

@@ -42,7 +42,7 @@ bdrv_query_image_info(BlockDriverState *bs, ImageInfo **p_info, bool flat,
                       bool skip_implicit_filters, Error **errp);
 void GRAPH_RDLOCK
 bdrv_query_block_graph_info(BlockDriverState *bs, BlockGraphInfo **p_info,
-                            Error **errp);
+                            bool limits, Error **errp);
 
 void bdrv_snapshot_dump(QEMUSnapshotInfo *sn);
 void bdrv_image_info_specific_dump(ImageInfoSpecific *info_spec,

qemu-img-cmds.hx

@@ -66,9 +66,9 @@ SRST
 ERST
 
 DEF("info", img_info,
-    "info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [-U] filename")
+    "info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [--limits] [-U] filename")
 SRST
-.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
+.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-U] FILENAME
 ERST
 
 DEF("map", img_map,

qemu-img.c

@@ -86,6 +86,7 @@ enum {
     OPTION_BITMAPS = 275,
     OPTION_FORCE = 276,
     OPTION_SKIP_BROKEN = 277,
+    OPTION_LIMITS = 278,
 };
 
 typedef enum OutputFormat {
@@ -3002,7 +3003,8 @@ static gboolean str_equal_func(gconstpointer a, gconstpointer b)
 static BlockGraphInfoList *collect_image_info_list(bool image_opts,
                                                    const char *filename,
                                                    const char *fmt,
-                                                   bool chain, bool force_share)
+                                                   bool chain, bool limits,
+                                                   bool force_share)
 {
     BlockGraphInfoList *head = NULL;
     BlockGraphInfoList **tail = &head;
@@ -3039,7 +3041,7 @@ static BlockGraphInfoList *collect_image_info_list(bool image_opts,
          * the chain manually here.
          */
         bdrv_graph_rdlock_main_loop();
-        bdrv_query_block_graph_info(bs, &info, &err);
+        bdrv_query_block_graph_info(bs, &info, limits, &err);
         bdrv_graph_rdunlock_main_loop();
 
         if (err) {
@@ -3088,6 +3090,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
     BlockGraphInfoList *list;
     bool image_opts = false;
     bool force_share = false;
+    bool limits = false;
 
     fmt = NULL;
     for(;;) {
@@ -3097,6 +3100,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
             {"image-opts", no_argument, 0, OPTION_IMAGE_OPTS},
             {"backing-chain", no_argument, 0, OPTION_BACKING_CHAIN},
             {"force-share", no_argument, 0, 'U'},
+            {"limits", no_argument, 0, OPTION_LIMITS},
             {"output", required_argument, 0, OPTION_OUTPUT},
             {"object", required_argument, 0, OPTION_OBJECT},
             {0, 0, 0, 0}
@@ -3119,6 +3123,8 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
 "     display information about the backing chain for copy-on-write overlays\n"
 "  -U, --force-share\n"
 "     open image in shared mode for concurrent access\n"
+"  --limits\n"
+"     show detected block limits (may depend on options, e.g. cache mode)\n"
 "  --output human|json\n"
 "     specify output format (default: human)\n"
 "  --object OBJDEF\n"
@@ -3140,6 +3146,9 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
         case 'U':
             force_share = true;
             break;
+        case OPTION_LIMITS:
+            limits = true;
+            break;
         case OPTION_OUTPUT:
             output_format = parse_output_format(argv[0], optarg);
             break;
@@ -3156,7 +3165,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
     filename = argv[optind++];
 
     list = collect_image_info_list(image_opts, filename, fmt, chain,
-                                   force_share);
+                                   limits, force_share);
     if (!list) {
         return 1;
     }