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

[Max Reitz]

On 17.03.20 19:16, 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 <address@hidden>
> ---
>  block/qcow2.h | 120 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 120 insertions(+)
> 
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 9611efbc52..52865787ee 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h

[...]

> @@ -447,6 +456,33 @@ typedef struct QCowL2Meta
>      QLIST_ENTRY(QCowL2Meta) next_in_flight;
>  } QCowL2Meta;
>  
> +/*
> + * In images with standard L2 entries all clusters are treated as if
> + * they had one subcluster so QCow2ClusterType and QCow2SubclusterType
> + * can be mapped to each other and have the exact same meaning
> + * (QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC cannot happen in these images).
> + *
> + * In images with extended L2 entries QCow2ClusterType refers to the
> + * complete cluster and QCow2SubclusterType to each of the individual
> + * subclusters, so there are several possible combinations:
> + *
> + *     |--------------+---------------------------|
> + *     | Cluster type | Possible subcluster types |
> + *     |--------------+---------------------------|
> + *     | UNALLOCATED  |         UNALLOCATED_PLAIN |
> + *     |              |                ZERO_PLAIN |
> + *     |--------------+---------------------------|
> + *     | NORMAL       |         UNALLOCATED_ALLOC |
> + *     |              |                ZERO_ALLOC |
> + *     |              |                    NORMAL |
> + *     |--------------+---------------------------|
> + *     | COMPRESSED   |                COMPRESSED |
> + *     |--------------+---------------------------|
> + *
> + * QCOW2_SUBCLUSTER_INVALID means that the L2 entry is incorrect and
> + * the image should be marked corrupt.
> + */
> +

Oh, a welcome addition! :)

[...]

> @@ -632,6 +678,80 @@ static inline QCow2ClusterType 
> qcow2_get_cluster_type(BlockDriverState *bs,

[...]

> +/*
> + * In an image without subsclusters @l2_bitmap is ignored and
> + * @sc_index must be 0.
> + */
> +static inline
> +QCow2SubclusterType qcow2_get_subcluster_type(BlockDriverState *bs,
> +                                              uint64_t l2_entry,
> +                                              uint64_t l2_bitmap,
> +                                              unsigned sc_index)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    QCow2ClusterType type = qcow2_get_cluster_type(bs, l2_entry);
> +    assert(sc_index < s->subclusters_per_cluster);
> +
> +    if (has_subclusters(s)) {
> +        bool sc_zero  = l2_bitmap & QCOW_OFLAG_SUB_ZERO(sc_index);
> +        bool sc_alloc = l2_bitmap & QCOW_OFLAG_SUB_ALLOC(sc_index);
> +        switch (type) {
> +        case QCOW2_CLUSTER_COMPRESSED:
> +            return QCOW2_SUBCLUSTER_COMPRESSED;

Why did you drop the check that l2_bitmap == 0 here?

Max

> +        case QCOW2_CLUSTER_ZERO_PLAIN:
> +        case QCOW2_CLUSTER_ZERO_ALLOC:
> +            return QCOW2_SUBCLUSTER_INVALID;
> +        case QCOW2_CLUSTER_NORMAL:
> +            if (!sc_zero && !sc_alloc) {
> +                return QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC;
> +            } else if (!sc_zero && sc_alloc) {
> +                return QCOW2_SUBCLUSTER_NORMAL;
> +            } else if (sc_zero && !sc_alloc) {
> +                return QCOW2_SUBCLUSTER_ZERO_ALLOC;
> +            } else { /* sc_zero && sc_alloc */
> +                return QCOW2_SUBCLUSTER_INVALID;
> +            }
> +        case QCOW2_CLUSTER_UNALLOCATED:
> +            if (!sc_zero && !sc_alloc) {
> +                return QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN;
> +            } else if (!sc_zero && sc_alloc) {
> +                return QCOW2_SUBCLUSTER_INVALID;
> +            } else if (sc_zero && !sc_alloc) {
> +                return QCOW2_SUBCLUSTER_ZERO_PLAIN;
> +            } else { /* sc_zero && sc_alloc */
> +                return QCOW2_SUBCLUSTER_INVALID;
> +            }
> +        default:
> +            g_assert_not_reached();
> +        }
> +    } else {
> +        return qcow2_cluster_to_subcluster_type(type);
> +    }
> +}
> +
>  /* Check whether refcounts are eager or lazy */
>  static inline bool qcow2_need_accurate_refcounts(BDRVQcow2State *s)
>  {
>

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