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Re: [Qemu-devel] [PATCH for 1.4] block/vpc: Fix size calculation
From: |
Jeff Cody |
Subject: |
Re: [Qemu-devel] [PATCH for 1.4] block/vpc: Fix size calculation |
Date: |
Mon, 11 Feb 2013 14:38:37 -0500 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Mon, Feb 11, 2013 at 09:54:56AM +0100, Kevin Wolf wrote:
> Am 08.02.2013 18:43, schrieb Stefan Weil:
> > Am 08.02.2013 13:14, schrieb Jeff Cody:
> >> 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"?
> >
> > Linux is a modern OS per definition :-)
> >
> > I made an additional test with Knoppix (from December 2006),
> > and it worked as expected.
>
> So I'm afraid that the reason isn't the guest OS but the Virtual PC
> version. I guess supporting newer versions of it is more important, so
> if Jeff's testing passes, I'm not against taking the patch, even though
> I don't feel completely comfortable about it. But I don't see how we
> could support both ways at the same time.
>
I've done some testing back and forth between VPC, Win Server 2012
Hyper-V, and qemu/kvm with and without this patch applied.
The short: this patch will break Virtual PC VHD compatibility (as it
exists under Win7), but allow compatibility for VHD files created by
Hyper-V (at least under Server 2012).
VPC and Server 2012 Hyper-V are also incompatible with each other,
when it comes to VHD files, if that is any consolation.
I used sha1sums between the different combination of VHD files filled to
capacity with random data under VPC, qemu/kvm (with & without the
patch), and Hyper-V. Without this patch we are an exact math with
VPC, and with this patch we are an exact match with Hyper-V.
However, perhaps we can support both, depending on who created the VHD
file. There is a "Creator Application" field in the VHD header - for
the VPC-created VHD file, that field is ASCII "vpc ". For the Hyper-V
created VHD file, that field is ASCII "win ". If we see the field as
"vpc ", we could default to using the CHS geometry found in the
header - otherwise, do as this patch does, and go by the virtual disk
size.
Jeff
Re: [Qemu-devel] [PATCH for 1.4] block/vpc: Fix size calculation, Anthony Liguori, 2013/02/11