qcow2: Add offset_to_l1_index()

Similar to offset_to_l2_index(), this function returns the index in
the L1 table for a given guest offset. This is only used in a couple
of places and it's not a particularly complex calculation, but it
makes the code a bit more readable.

Although in the qcow2_get_cluster_offset() case the old code was
taking advantage of the l1_bits variable, we're going to get rid of
the other uses of l1_bits in a later patch anyway, so it doesn't make
sense to keep it just for this.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: a5f626fed526b7459a0425fad06d823d18df8522.1517840877.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/qcow2-cluster.c

@@ -540,7 +540,7 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
 
     /* seek to the l2 offset in the l1 table */
 
-    l1_index = offset >> l1_bits;
+    l1_index = offset_to_l1_index(s, offset);
     if (l1_index >= s->l1_size) {
         type = QCOW2_CLUSTER_UNALLOCATED;
         goto out;
@@ -663,7 +663,7 @@ static int get_cluster_table(BlockDriverState *bs, uint64_t offset,
 
     /* seek to the l2 offset in the l1 table */
 
-    l1_index = offset >> (s->l2_bits + s->cluster_bits);
+    l1_index = offset_to_l1_index(s, offset);
     if (l1_index >= s->l1_size) {
         ret = qcow2_grow_l1_table(bs, l1_index + 1, false);
         if (ret < 0) {

block/qcow2.h

@@ -463,6 +463,11 @@ static inline int64_t size_to_l1(BDRVQcow2State *s, int64_t size)
     return (size + (1ULL << shift) - 1) >> shift;
 }
 
+static inline int offset_to_l1_index(BDRVQcow2State *s, uint64_t offset)
+{
+    return offset >> (s->l2_bits + s->cluster_bits);
+}
+
 static inline int offset_to_l2_index(BDRVQcow2State *s, int64_t offset)
 {
     return (offset >> s->cluster_bits) & (s->l2_size - 1);