Re: [RFC PATCH v3 11/27] qcow2: Add QCow2SubclusterType and qcow2_get_subcluster_type()

[Max Reitz <mreitz@redhat.com>]

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On 22.12.19 12:36, 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 | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 92 insertions(+)

With the comment style fixed as now required by the coding style (/* and
*/ on separate lines), and regardless of the bit ordering:

Reviewed-by: Max Reitz <mreitz@redhat.com>


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