Re: [Qemu-devel] [PATCH for 1.4] block/vpc: Fix size calculation

[Stefan Hajnoczi]

On Fri, Feb 08, 2013 at 07:14:33AM -0500, Jeff Cody wrote:
> On Fri, Feb 08, 2013 at 09:38:47AM +0100, Kevin Wolf wrote:
> > Am 07.02.2013 20:26, schrieb Stefan Weil:
> > > From: Stefan Weil <address@hidden(none)>
> > > 
> > > The size calculated from the CHS values is not the real image (disk) size,
> > > but usually a smaller value. This is caused by rounding effects.
> > > 
> > > Only older operating systems use CHS. Such guests won't be able to use
> > > the whole disk. All modern operating systems use the real size.
> > > 
> > > This patch fixes https://bugs.launchpad.net/qemu/+bug/1105670/.
> > > 
> > > Signed-off-by: Stefan Weil <address@hidden>
> > > ---
> > > 
> > > This is a rebased extract from my patch series for block/vpc.c.
> > > It's the minimum needed to fix the open bug for QEMU 1.4.
> > > 
> > > The rest of the series can be discussed and applied after 1.4.
> > > 
> > > Regards
> > > 
> > > Stefan W.
> > > 
> > > PS. Please excuse a previous personal mail which I had sent
> > > with a wrong signature and without addressing qemu-devel.
> > > 
> > >  block/vpc.c |   14 +++++++++-----
> > >  1 file changed, 9 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/block/vpc.c b/block/vpc.c
> > > index 82229ef..b4ff564 100644
> > > --- a/block/vpc.c
> > > +++ b/block/vpc.c
> > > @@ -34,6 +34,8 @@
> > >  
> > >  #define HEADER_SIZE 512
> > >  
> > > +#define VHD_SECTOR_SIZE 512
> > > +
> > >  //#define CACHE
> > >  
> > >  enum vhd_type {
> > > @@ -204,11 +206,13 @@ static int vpc_open(BlockDriverState *bs, int flags)
> > >      /* Write 'checksum' back to footer, or else will leave it with zero. 
> > > */
> > >      footer->checksum = be32_to_cpu(checksum);
> > >  
> > > -    // The visible size of a image in Virtual PC depends on the geometry
> > > -    // rather than on the size stored in the footer (the size in the 
> > > footer
> > > -    // is too large usually)
> > > -    bs->total_sectors = (int64_t)
> > > -        be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl;
> > > +    /* The visible size of a image in Virtual PC depends on the guest:
> > > +     * QEMU and other emulators report the real size (here in sectors).
> > > +     * All modern operating systems use this real size.
> > > +     * Very old operating systems use CHS values to calculate the total 
> > > size.
> > > +     * This calculated size is usually smaller than the real size.
> > > +     */
> > > +    bs->total_sectors = be64_to_cpu(footer->size) / VHD_SECTOR_SIZE;
> > 
> > It's unfortunate that I don't have my old Virtual PC installation around
> > any more so I could prove that you're wrong for at least some versions.
> > Or does a Linux of 2009 already count as "very old"?
> > 
> > If we want to commit this - and I still feel uncomfortable about it -
> > then maybe it's best to remove the comment altogether instead of making
> > such claims. The new code is the intuitively expected one anyway.
> > 
> > Kevin
> 
> Kevin,
> 
> I can test this on Virtual PC on a Win 7 install, as well as
> Hyper-V (I've already tested the equivalent of this patch on Hyper-V
> on Windows Server 2012).  I'll do some testing today with this - if
> you have anything in particular you want me to look at, just let me
> know.

Hi Jeff,
Thanks for testing this on your Virtual PC and Hyper-V setup.  I'll
merge this patch once you have confirmed your results.

Stefan