qcow2: Convert qcow2_get_cluster_offset() into qcow2_get_host_offset()

qcow2_get_cluster_offset() takes an (unaligned) guest offset and
returns the (aligned) offset of the corresponding cluster in the qcow2
image.

In practice none of the callers need to know where the cluster starts
so this patch makes the function calculate and return the final host
offset directly. The function is also renamed accordingly.

There is a pre-existing exception with compressed clusters: in this
case the function returns the complete cluster descriptor (containing
the offset and size of the compressed data). This does not change with
this patch but it is now documented.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <ffae6cdc5ca8950e8280ac0f696dcc376cb07095.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 23 insertions, 18 deletions

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 550850b264..71ddb0c462 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -496,10 +496,15 @@ static int coroutine_fn do_perform_cow_write(BlockDriverState *bs,
 
 
 /*
- * get_cluster_offset
+ * get_host_offset
  *
- * For a given offset of the virtual disk, find the cluster type and offset in
- * the qcow2 file. The offset is stored in *cluster_offset.
+ * For a given offset of the virtual disk find the equivalent host
+ * offset in the qcow2 file and store it in *host_offset. Neither
+ * offset needs to be aligned to a cluster boundary.
+ *
+ * If the cluster is unallocated then *host_offset will be 0.
+ * If the cluster is compressed then *host_offset will contain the
+ * complete compressed cluster descriptor.
  *
  * On entry, *bytes is the maximum number of contiguous bytes starting at
  * offset that we are interested in.
@@ -511,12 +516,12 @@ static int coroutine_fn do_perform_cow_write(BlockDriverState *bs,
  * Returns the cluster type (QCOW2_CLUSTER_*) on success, -errno in error
  * cases.
  */
-int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
-                             unsigned int *bytes, uint64_t *cluster_offset)
+int qcow2_get_host_offset(BlockDriverState *bs, uint64_t offset,
+                          unsigned int *bytes, uint64_t *host_offset)
 {
     BDRVQcow2State *s = bs->opaque;
     unsigned int l2_index;
-    uint64_t l1_index, l2_offset, *l2_slice;
+    uint64_t l1_index, l2_offset, *l2_slice, l2_entry;
     int c;
     unsigned int offset_in_cluster;
     uint64_t bytes_available, bytes_needed, nb_clusters;
@@ -537,7 +542,7 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
         bytes_needed = bytes_available;
     }
 
-    *cluster_offset = 0;
+    *host_offset = 0;
 
     /* seek to the l2 offset in the l1 table */
 
@@ -570,7 +575,7 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
     /* find the cluster offset for the given disk offset */
 
     l2_index = offset_to_l2_slice_index(s, offset);
-    *cluster_offset = be64_to_cpu(l2_slice[l2_index]);
+    l2_entry = be64_to_cpu(l2_slice[l2_index]);
 
     nb_clusters = size_to_clusters(s, bytes_needed);
     /* bytes_needed <= *bytes + offset_in_cluster, both of which are unsigned
@@ -578,7 +583,7 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
      * true */
     assert(nb_clusters <= INT_MAX);
 
-    type = qcow2_get_cluster_type(bs, *cluster_offset);
+    type = qcow2_get_cluster_type(bs, l2_entry);
     if (s->qcow_version < 3 && (type == QCOW2_CLUSTER_ZERO_PLAIN ||
                                 type == QCOW2_CLUSTER_ZERO_ALLOC)) {
         qcow2_signal_corruption(bs, true, -1, -1, "Zero cluster entry found"
@@ -599,42 +604,42 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
         }
         /* Compressed clusters can only be processed one by one */
         c = 1;
-        *cluster_offset &= L2E_COMPRESSED_OFFSET_SIZE_MASK;
+        *host_offset = l2_entry & L2E_COMPRESSED_OFFSET_SIZE_MASK;
         break;
     case QCOW2_CLUSTER_ZERO_PLAIN:
     case QCOW2_CLUSTER_UNALLOCATED:
         /* how many empty clusters ? */
         c = count_contiguous_clusters_unallocated(bs, nb_clusters,
                                                   &l2_slice[l2_index], type);
-        *cluster_offset = 0;
         break;
     case QCOW2_CLUSTER_ZERO_ALLOC:
-    case QCOW2_CLUSTER_NORMAL:
+    case QCOW2_CLUSTER_NORMAL: {
+        uint64_t host_cluster_offset = l2_entry & L2E_OFFSET_MASK;
+        *host_offset = host_cluster_offset + offset_in_cluster;
         /* how many allocated clusters ? */
         c = count_contiguous_clusters(bs, nb_clusters, s->cluster_size,
                                       &l2_slice[l2_index], QCOW_OFLAG_ZERO);
-        *cluster_offset &= L2E_OFFSET_MASK;
-        if (offset_into_cluster(s, *cluster_offset)) {
+        if (offset_into_cluster(s, host_cluster_offset)) {
             qcow2_signal_corruption(bs, true, -1, -1,
                                     "Cluster allocation offset %#"
                                     PRIx64 " unaligned (L2 offset: %#" PRIx64
-                                    ", L2 index: %#x)", *cluster_offset,
+                                    ", L2 index: %#x)", host_cluster_offset,
                                     l2_offset, l2_index);
             ret = -EIO;
             goto fail;
         }
-        if (has_data_file(bs) && *cluster_offset != offset - offset_in_cluster)
-        {
+        if (has_data_file(bs) && *host_offset != offset) {
             qcow2_signal_corruption(bs, true, -1, -1,
                                     "External data file host cluster offset %#"
                                     PRIx64 " does not match guest cluster "
                                     "offset: %#" PRIx64
-                                    ", L2 index: %#x)", *cluster_offset,
+                                    ", L2 index: %#x)", host_cluster_offset,
                                     offset - offset_in_cluster, l2_index);
             ret = -EIO;
             goto fail;
         }
         break;
+    }
     default:
         abort();
     }