[Qemu-block] [PATCH] qcow2: Clarify that compressed cluster offset requires shift

[Eric Blake]

The specs for the host cluster offset of a compressed cluster
were not clear that the offset is in terms of sectors, and requires
a shift by 9 to be a byte offset.  Add some more text to make the
interpretation obvious.

CC: address@hidden
Signed-off-by: Eric Blake <address@hidden>
---
 docs/specs/qcow2.txt | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 80cdfd0..7049415 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -323,11 +323,16 @@ Standard Cluster Descriptor:

 Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):

-    Bit  0 -  x:    Host cluster offset. This is usually _not_ aligned to a
-                    cluster boundary!
+    Bit  0 -  x:    Bits 9-(x+9) of host cluster offset. This is
+                    usually _not_ aligned to a cluster boundary!

        x+1 - 61:    Compressed size of the images in sectors of 512 bytes

+The bits of the host cluster offset not specified in the cluster descriptor
+are 0 (bits 0-8 are obvious because a 512-byte sector is the smallest
+addressable unit, while bits 56-63 implies that a qcow2 file cannot exceed
+2^56 bytes in size).
+
 If a cluster is unallocated, read requests shall read the data from the backing
 file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
 no backing file or the backing file is smaller than the image, they shall read
-- 
2.5.0