[v7,15/32] qcow2: Add qcow2_get_subcluster_range_type()

Commit Message

Alberto Garcia May 25, 2020, 6:08 p.m. UTC

There are situations in which we want to know how many contiguous
subclusters of the same type there are in a given cluster. This can be
done by simply iterating over the subclusters and repeatedly calling
qcow2_get_subcluster_type() for each one of them.

However once we determined the type of a subcluster we can check the
rest efficiently by counting the number of adjacent ones (or zeroes)
in the bitmap. This is what this function does.

Signed-off-by: Alberto Garcia <[e-mail]>
---
 block/qcow2-cluster.c | 51 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

Comments

Eric Blake May 26, 2020, 9:48 p.m. UTC | #1

On 5/25/20 1:08 PM, Alberto Garcia wrote:
> There are situations in which we want to know how many contiguous
> subclusters of the same type there are in a given cluster. This can be
> done by simply iterating over the subclusters and repeatedly calling
> qcow2_get_subcluster_type() for each one of them.
> 
> However once we determined the type of a subcluster we can check the
> rest efficiently by counting the number of adjacent ones (or zeroes)
> in the bitmap. This is what this function does.
> 
> Signed-off-by: Alberto Garcia <[e-mail]>
> ---
>   block/qcow2-cluster.c | 51 +++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 51 insertions(+)
> 

> +    if (*type == QCOW2_SUBCLUSTER_INVALID) {
> +        return -EINVAL;
> +    } else if (!has_subclusters(s) || *type == QCOW2_SUBCLUSTER_COMPRESSED) {
> +        return s->subclusters_per_cluster - sc_from;
> +    }
> +
> +    switch (*type) {
> +    case QCOW2_SUBCLUSTER_NORMAL:
> +        val = l2_bitmap | QCOW_OFLAG_SUB_ALLOC_RANGE(0, sc_from);
> +        return cto32(val) - sc_from;

Slick.

Reviewed-by: Eric Blake <[e-mail]>

Patch

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 8b2fc550b7..32dc6e75e3 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -375,6 +375,57 @@  fail:
     return ret;
 }
 
+/*
+ * For a given L2 entry, count the number of contiguous subclusters of
+ * the same type starting from @sc_from. Compressed clusters are
+ * treated as if they were divided into subclusters of size
+ * s->subcluster_size.
+ *
+ * Return the number of contiguous subclusters and set @type to the
+ * subcluster type.
+ *
+ * If the L2 entry is invalid return -errno and set @type to
+ * QCOW2_SUBCLUSTER_INVALID.
+ */
+G_GNUC_UNUSED
+static int qcow2_get_subcluster_range_type(BlockDriverState *bs,
+                                           uint64_t l2_entry,
+                                           uint64_t l2_bitmap,
+                                           unsigned sc_from,
+                                           QCow2SubclusterType *type)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint32_t val;
+
+    *type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_from);
+
+    if (*type == QCOW2_SUBCLUSTER_INVALID) {
+        return -EINVAL;
+    } else if (!has_subclusters(s) || *type == QCOW2_SUBCLUSTER_COMPRESSED) {
+        return s->subclusters_per_cluster - sc_from;
+    }
+
+    switch (*type) {
+    case QCOW2_SUBCLUSTER_NORMAL:
+        val = l2_bitmap | QCOW_OFLAG_SUB_ALLOC_RANGE(0, sc_from);
+        return cto32(val) - sc_from;
+
+    case QCOW2_SUBCLUSTER_ZERO_PLAIN:
+    case QCOW2_SUBCLUSTER_ZERO_ALLOC:
+        val = (l2_bitmap | QCOW_OFLAG_SUB_ZERO_RANGE(0, sc_from)) >> 32;
+        return cto32(val) - sc_from;
+
+    case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
+    case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
+        val = ((l2_bitmap >> 32) | l2_bitmap)
+            & ~QCOW_OFLAG_SUB_ALLOC_RANGE(0, sc_from);
+        return ctz32(val) - sc_from;
+
+    default:
+        g_assert_not_reached();
+    }
+}
+
 /*
  * Checks how many clusters in a given L2 slice are contiguous in the image
  * file. As soon as one of the flags in the bitmask stop_flags changes compared