Re: [Qemu-devel] Question about total_sectors in block/vpc.c

[Christoph Hellwig <hch@lst.de>]

On Sat, Apr 09, 2011 at 09:05:41PM +0100, Stefan Hajnoczi wrote:
> On Sat, Apr 9, 2011 at 5:51 PM, Lyu Mitnick <mitnick.lyu@gmail.com> wrote:
> > Hell all,
> > I have take a look of block/vpc.c and meet a question in vpc_create().?At
> > the line
> > 550, the code is:
> > total_sectors = options->value.n / 512;
> > I am wondering whether the size between?total_sectors * 512
> > and?options->value.n
> > would be discard.
> 
> Yes, it rounds down.  This reflects the assumption that a block device
> cannot be addressed below 512 byte sectors.  Because of this block
> devices size must be a multiple of 512 bytes.
> 
> I think a reasonable protection would be to have block.c:bdrv_create()
> fail if size is not a multiple of BDRV_SECTOR_SIZE.  This way other
> image formats are protected too.

There are block devices that aren't alignment to 512 bytes.  Audio CDROMs are
the most prominent example, or AS/400 disks.  I don't think these matter for
emulation, but if we'd ever implement DIF/DIX emulation inside qemu we'd
have to store the protection information somewhere.  It still wouldn't
work with existing disk format, so adding the above check into the formats
bdrv_create routines sounds fine, but doing it in the core block code
might not be an overly smart idea.