Re: [Qemu-devel] [PATCH v2 07/21] qcow2: Helper function for refcount modification

[Eric Blake <eblake@redhat.com>]

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On 11/14/2014 06:06 AM, Max Reitz wrote:
> Since refcounts do not always have to be a uint16_t, all refcount blocks
> and arrays in memory should not have a specific type (thus they become
> pointers to void) and for accessing them, two helper functions are used
> (a getter and a setter). Those functions are called indirectly through
> function pointers in the BDRVQcowState so they may later be exchanged
> for different refcount orders.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-refcount.c | 128 ++++++++++++++++++++++++++++++-------------------
>  block/qcow2.h          |   8 ++++
>  2 files changed, 87 insertions(+), 49 deletions(-)
> 

> @@ -1216,7 +1249,7 @@ enum {
>   * error occurred.
>   */
>  static int check_refcounts_l2(BlockDriverState *bs, BdrvCheckResult *res,
> -    uint16_t **refcount_table, int64_t *refcount_table_size, int64_t l2_offset,
> +    void **refcount_table, int64_t *refcount_table_size, int64_t l2_offset,
>      int flags)
>  {

Might be worth fixing the indentation here in addition to all the other
places you adjusted.  But that's minor.

> @@ -1933,17 +1967,13 @@ write_refblocks:
>              goto fail;
>          }
>  
> -        on_disk_refblock = qemu_blockalign0(bs->file, s->cluster_size);
> -        for (i = 0; i < s->refcount_block_size &&
> -                    refblock_start + i < *nb_clusters; i++)
> -        {
> -            on_disk_refblock[i] =
> -                cpu_to_be16((*refcount_table)[refblock_start + i]);
> -        }
> +        /* The size of *refcount_table is always cluster-aligned, therefore the
> +         * write operation will not overflow */
> +        on_disk_refblock = (void *)((uintptr_t)*refcount_table +
> +                                    (refblock_index << s->refcount_block_bits));

Here is where you are relying on the guarantee that you added in 6/21,
which is why I ask for that one to mention it.

Nice reduction of a bounce buffer, by the way :)  Worth mentioning in
the commit message as an intentional part of this commit?

> @@ -2087,7 +2117,7 @@ int qcow2_check_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
>          /* Because the old reftable has been exchanged for a new one the
>           * references have to be recalculated */
>          rebuild = false;
> -        memset(refcount_table, 0, nb_clusters * sizeof(uint16_t));
> +        memset(refcount_table, 0, nb_clusters * s->refcount_bits / 8);

Phew; we're safe that this won't overflow; and good that you do the *
first (if you did the /8 first, it would fail for sub-byte refcounts).

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

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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