Re: [PATCH v7 14/32] qcow2: Add QCow2SubclusterType and qcow2_get_subcluster_type()

[Eric Blake <eblake@redhat.com>]

On 5/25/20 1:08 PM, Alberto Garcia wrote:
> This patch adds QCow2SubclusterType, which is the subcluster-level
> version of QCow2ClusterType. All QCOW2_SUBCLUSTER_* values have the
> the same meaning as their QCOW2_CLUSTER_* equivalents (when they
> exist). See below for details and caveats.
> 
> In images without extended L2 entries clusters are treated as having
> exactly one subcluster so it is possible to replace one data type with
> the other while keeping the exact same semantics.
> 
> With extended L2 entries there are new possible values, and every
> subcluster in the same cluster can obviously have a different
> QCow2SubclusterType so functions need to be adapted to work on the
> subcluster level.
> 
> There are several things that have to be taken into account:
> 
>    a) QCOW2_SUBCLUSTER_COMPRESSED means that the whole cluster is
>       compressed. We do not support compression at the subcluster
>       level.
> 
>    b) There are two different values for unallocated subclusters:
>       QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN which means that the whole
>       cluster is unallocated, and QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC
>       which means that the cluster is allocated but the subcluster is
>       not. The latter can only happen in images with extended L2
>       entries.
> 
>    c) QCOW2_SUBCLUSTER_INVALID is used to detect the cases where an L2
>       entry has a value that violates the specification. The caller is
>       responsible for handling these situations.
> 
>       To prevent compatibility problems with images that have invalid
>       values but are currently being read by QEMU without causing side
>       effects, QCOW2_SUBCLUSTER_INVALID is only returned for images
>       with extended L2 entries.
> 
> qcow2_cluster_to_subcluster_type() is added as a separate function
> from qcow2_get_subcluster_type(), but this is only temporary and both
> will be merged in a subsequent patch.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>   block/qcow2.h | 126 +++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 125 insertions(+), 1 deletion(-)
> 
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 5c6bf48c7a..27dbcbc502 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -80,6 +80,21 @@
>   
>   #define QCOW_EXTL2_SUBCLUSTERS_PER_CLUSTER 32
>   
> +/* The subcluster X [0..31] is allocated */
> +#define QCOW_OFLAG_SUB_ALLOC(X)   (1ULL << (X))
> +/* The subcluster X [0..31] reads as zeroes */
> +#define QCOW_OFLAG_SUB_ZERO(X)    (QCOW_OFLAG_SUB_ALLOC(X) << 32)
> +/* Subclusters [X, Y) (0 <= X <= Y <= 32) are allocated */

As you are now using a half-open range, should this be:
  (0 <= X < Y <= 32)

> +#define QCOW_OFLAG_SUB_ALLOC_RANGE(X, Y) \
> +    (QCOW_OFLAG_SUB_ALLOC(Y) - QCOW_OFLAG_SUB_ALLOC(X))

with <= instead of <, then it is impossible to distinguish between 
QCOW_OFLAG_SUB_ALLOC_RANGE(0,0) and QCOW_OFLAG_SUB_ALLOC_RANGE(31,31) 
which both resolve to 0.  I guess it depends on whether the later uses 
of this macro require a non-zero mask ('X < Y') or tolerate the corner 
case of no subclusters selected ('X <= Y').

> +/* Subclusters [X, Y) (0 <= X <= Y <= 32) read as zeroes */
> +#define QCOW_OFLAG_SUB_ZERO_RANGE(X, Y) \
> +    (QCOW_OFLAG_SUB_ALLOC_RANGE(X, Y) << 32)
> +/* L2 entry bitmap with all allocation bits set */
> +#define QCOW_L2_BITMAP_ALL_ALLOC  (QCOW_OFLAG_SUB_ALLOC_RANGE(0, 32))
> +/* L2 entry bitmap with all "read as zeroes" bits set */
> +#define QCOW_L2_BITMAP_ALL_ZEROES (QCOW_OFLAG_SUB_ZERO_RANGE(0, 32))
> +

Fixing the comment (if necessary) does not change the code, and the rest 
of this patch is fine, so:

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org