Re: [PATCH v4 13/30] qcow2: Add QCow2SubclusterType and qcow2_get_subcluster_type()

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

17.03.2020 21:16, 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>


Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

-- 
Best regards,
Vladimir