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

[Lyu Mitnick <mitnick.lyu@gmail.com>]

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Hello Christoph, Stefan

I am wondering whether the problem occurred that value of BDRV_SECTOR_SIZE
is
a macro constant (defined at block.h). This problem could be fixed if we use
variable
instead of macro to implement BDRV_SECTOR_SIZE. Each block device may
reassign
the value if needed. Is it right??

Thanks a lot

2011/4/12 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.
>
>
Mitnick

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