Re: [Qemu-devel] [PATCH 10/17] block: define get_block_status return value

[Fam Zheng <famz@redhat.com>]

On Thu, 07/04 23:10, Peter Lieven wrote:
> 
> Am 04.07.2013 um 10:13 schrieb Paolo Bonzini <pbonzini@redhat.com>:
> 
> > Il 03/07/2013 23:04, Peter Lieven ha scritto:
> >>>> Define the return value of get_block_status.  Bits 0, 1, 2 and 8-62
> >>>> are valid; bit 63 (the sign bit) is reserved for errors.  Bits 3-7
> >>>> are left for future extensions.
> >> Is Bit 8 not also reserved for future use? BDRV_SECTOR_BITS is 9.
> > 
> > Right.
> > 
> >> Can you explain which information is exactly returned in Bits 9-62?
> > 
> > Bits 9-62 are the offset at which the data is stored in bs->file, they
> > are valid if bit 2 (BDRV_BLOCK_OFFSET_VALID) is 1.
> 
> Ok, so this is if bs->file is not linear?
> 
> If we return the offset into bs->file this would only make sense if the data
> at that position is raw and not encoded otherwise and if *pnum is limited
> to the size of the extend at that position, right?
Exactly.
> 
> I currently do not understand for what operation this info is needed.
Quoted from the cover letter:
  > One example usage is
  > (for non-compressed, non-encrypted images) to transform the metadata
  > into a Linux device-mapper setup, and make a qcow2 image available (for
  > read only) as a block device.  Another possible usage is to determine
  > the used areas of a file, and convert it in place to another format.

Thanks.

Fam