Re: [Qemu-devel] [PATCH v5 12/21] block: define get_block_status return value

[Paolo Bonzini]

Il 06/05/2014 15:08, Kevin Wolf ha scritto:
> > This is true, because bdrv_get_block_status does not ask the backing
> > file whether it is zero in the backing file.  Thus, reading from the
> > backing file does not guarantee to read zeroes.
>
> Aha, this paragraph is the key. :-)
>
> I assumed that bdrv_get_block_status() does ask the backing file, but
> apparently it doesn't. The current meaning of BDRV_BLOCK_ZERO is
> therefore not simply "sectors read as zero", but "sectors read as zero
> and this is determined by bs itself (mostly, except for the backing file
> size)".

Exactly.

> Is it a good idea to rely on this or would it be reasonable for
> bdrv_get_block_status() to look at the backing file for returning ZERO
> sometime in the future? I think that could be useful information in some
> cases.

It is reasonable, for example SCSI emulation could use it.  However, it 
would be a different API (like bdrv_is_allocated_above).  It would be 
very similar to get_block_status() in qemu-img.c.

Paolo