Re: [Qemu-devel] [PATCH 2/2] docs/qcow2: Correct refcount_block_entries

[Eric Blake <eblake@redhat.com>]

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On 08/29/2014 03:45 PM, Max Reitz wrote:
> A refblock entry may have a different size than 16 bits, it may even be
> smaller than a byte. Correct the refcount_block_entries calculation
> accordingly.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  docs/specs/qcow2.txt | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index cfbc8b0..531c478 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -183,7 +183,7 @@ blocks and are exactly one cluster in size.
>  Given a offset into the image file, the refcount of its cluster can be obtained
>  as follows:
>  
> -    refcount_block_entries = (cluster_size / sizeof(uint16_t))
> +    refcount_block_entries = (cluster_size / (refcount_bits / 8))
>  


Consider refcount_order == 0 (that is, no shared blocks, ALL blocks have
at most refcount 1).  Then, refcount_bits is (1 << 0) == 1.  But 1/8 in
integer math truncates to 0 (oops, division by zero is undefined); when
in reality, the expression you want here is (cluster_size * 8 /
refcount_bits).

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


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