Re: [Qemu-devel] [Qemu-block] [PATCH] qcow2: Clarify that compressed cluster offset requires shift

[Max Reitz <mreitz@redhat.com>]

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On 29.02.2016 16:11, Eric Blake wrote:
> The specs for the host cluster offset of a compressed cluster
> were not clear that the offset is in terms of sectors, and requires
> a shift by 9 to be a byte offset.  Add some more text to make the
> interpretation obvious.
> 
> CC: mgreger@cinci.rr.com
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  docs/specs/qcow2.txt | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 80cdfd0..7049415 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -323,11 +323,16 @@ Standard Cluster Descriptor:
> 
>  Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
> 
> -    Bit  0 -  x:    Host cluster offset. This is usually _not_ aligned to a
> -                    cluster boundary!
> +    Bit  0 -  x:    Bits 9-(x+9) of host cluster offset. This is

I'd rather use one of "9..(x+9)", "[9, x+9]", "9 - (x+9)", "9—(x+9)" in
order to clarify that this is not a minus but a range.

Max

> +                    usually _not_ aligned to a cluster boundary!
> 
>         x+1 - 61:    Compressed size of the images in sectors of 512 bytes
> 
> +The bits of the host cluster offset not specified in the cluster descriptor
> +are 0 (bits 0-8 are obvious because a 512-byte sector is the smallest
> +addressable unit, while bits 56-63 implies that a qcow2 file cannot exceed
> +2^56 bytes in size).
> +
>  If a cluster is unallocated, read requests shall read the data from the backing
>  file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
>  no backing file or the backing file is smaller than the image, they shall read
> 



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