Re: [Qemu-devel] [PATCH v5 01/11] qcow2: Calculate refcount block entry count

[Benoît Canet]

On Fri, Aug 29, 2014 at 11:40:53PM +0200, Max Reitz wrote:
> The size of a refblock entry is (in theory) variable; calculate
> therefore the number of entries per refblock and the according bit shift
> (1 << x == entry count) when opening an image.
> 
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  block/qcow2.c | 2 ++
>  block/qcow2.h | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index f9e045f..172ad00 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -689,6 +689,8 @@ static int qcow2_open(BlockDriverState *bs, QDict 
> *options, int flags,
>  
>      s->l2_bits = s->cluster_bits - 3; /* L2 is always one cluster */
>      s->l2_size = 1 << s->l2_bits;
> +    s->refcount_block_bits = s->cluster_bits - (s->refcount_order - 3);

After carefull examination (s->refcount_order - 3) == REFCOUNT_SHIFT.
Why not also creating s->refcount_shift and make use of it in order to avoid
torturing the mind of the next reader.

Or simply recall in a comment that there is 2^3 bits in a byte.

Best regards

Benoît

> +    s->refcount_block_size = 1 << s->refcount_block_bits;
>      bs->total_sectors = header.size / 512;
>      s->csize_shift = (62 - (s->cluster_bits - 8));
>      s->csize_mask = (1 << (s->cluster_bits - 8)) - 1;
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 6aeb7ea..7c01fb7 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -222,6 +222,8 @@ typedef struct BDRVQcowState {
>      int l2_size;
>      int l1_size;
>      int l1_vm_state_index;
> +    int refcount_block_bits;
> +    int refcount_block_size;
>      int csize_shift;
>      int csize_mask;
>      uint64_t cluster_offset_mask;
> -- 
> 2.1.0
>