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

[Jeff Cody]

On Tue, Feb 12, 2013 at 03:47:38PM +0100, Stefan Weil wrote:
> Am 11.02.2013 22:20, schrieb Jeff Cody:
> > On Mon, Feb 11, 2013 at 03:14:36PM -0600, Anthony Liguori wrote:
> >> Paolo Bonzini <address@hidden> writes:
> >>
> >>> Il 11/02/2013 21:13, Anthony Liguori ha scritto:
> >>>> Applied.  Thanks.
> >>>>
> >>>> Regards,
> >>>>
> >>>> Anthony Liguori
> >>> I guess this was a mistake, please revert.
> >> No, it wasn't.  The patch was reviewed and tested.  What's the problem.
> >> It's proposed for 1.4 so it's now or never.
> >>
> >> I saw some discussion about improving the detection for the different
> >> varients of VHD...
> >>
> >> I don't mind reverting it, but just want to understand what the issue
> >> is.
> >>
> > Hi Anthony,
> >
> > It was tested, the problem is that this will break existing support
> > for Virtual PC VHD files under certain conditions (most likely
> > exporting from qemu to VPC, but I don't know if we could guarantee
> > that is the only way).
> >
> >
> > Jeff
> 
> Hi Jeff,
> 
> exporting from QEMU to VPCwill work because QEMU uses
> images where real size = CHS size, so those images work
> no matter whether the patch was applied or not.
> 
> See my separate mail for more reasoning.
> 
> I see no way how the patch could break a realistic test scenario,
> but I know such scenarios which are broken without the patch.
> 
> 
> Stefan
> 

Hi Stefan,

I should have been clearer when I said export - I am talking about
qemu modifying an existing VHD image created by VPC, and then trying
to use image in VPC again.

Here is a simple way to see how the proposed patch breaks current VHD
compatibility:

1.) Under VPC, define a new VM that boots from a live dvd iso (I used
a puppy linux iso because it was small and quick to test:
http://puppylinux.org). For the hard drive for the machine, define a
128GB dynamic drive.  (test.vhd).  You don't need to boot this VM yet.

2.) Now, under QEMU, boot again to a live iso or existing linux image,
and use the newly created test.vhd.  Create a single partition, using
the fdisk defaults for the maximum size.  Shutdown.

3.) Use that same VHD image and boot the live iso with your VPC
machine.  Try to mount the partition you created under qemu with the
guest in VPC.  It will fail to detect the partition, and complain
about invalid geometry.

I think your patch is definitely needed, don't get me wrong, and I can
verify that it works - for the bug report, I even came to the same
conclusion.  I just think it needs to be done such that we don't break
something that used to work, and testing shows that it does,
unfortunately.

Thanks,
Jeff