block: add subcluster_size field to BlockDriverInfo

This is going to be used in the subsequent commit as requests alignment
(in particular, during copy-on-read).  This value only makes sense for
the formats which support subclusters (currently QCOW2 only).  If this
field isn't set by driver's own bdrv_get_info() implementation, we
simply set it equal to the cluster size thus treating each cluster as
having a single subcluster.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230711172553.234055-2-andrey.drobyshev@virtuozzo.com>

block.c

@@ -6480,6 +6480,13 @@ int coroutine_fn bdrv_co_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
     }
     memset(bdi, 0, sizeof(*bdi));
     ret = drv->bdrv_co_get_info(bs, bdi);
+    if (bdi->subcluster_size == 0) {
+        /*
+         * If the driver left this unset, subclusters are not supported.
+         * Then it is safe to treat each cluster as having only one subcluster.
+         */
+        bdi->subcluster_size = bdi->cluster_size;
+    }
     if (ret < 0) {
         return ret;
     }

block/qcow2.c

@@ -5197,6 +5197,7 @@ qcow2_co_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
 {
     BDRVQcow2State *s = bs->opaque;
     bdi->cluster_size = s->cluster_size;
+    bdi->subcluster_size = s->subcluster_size;
     bdi->vm_state_offset = qcow2_vm_state_offset(s);
     bdi->is_dirty = s->incompatible_features & QCOW2_INCOMPAT_DIRTY;
     return 0;

include/block/block-common.h

@@ -132,6 +132,11 @@ typedef struct BlockZoneWps {
 typedef struct BlockDriverInfo {
     /* in bytes, 0 if irrelevant */
     int cluster_size;
+    /*
+     * A fraction of cluster_size, if supported (currently QCOW2 only); if
+     * disabled or unsupported, set equal to cluster_size.
+     */
+    int subcluster_size;
     /* offset at which the VM state can be saved (0 if not possible) */
     int64_t vm_state_offset;
     bool is_dirty;