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Re: [Qemu-devel] [RFC PATCH] vpc: Ignore geometry for large images

From: Charles Arnold
Subject: Re: [Qemu-devel] [RFC PATCH] vpc: Ignore geometry for large images
Date: Thu, 12 Feb 2015 10:18:35 -0700

>>> On 2/12/2015 at 03:23 AM, Kevin Wolf <address@hidden> wrote: 
> Am 12.02.2015 um 11:09 hat Peter Lieven geschrieben:
>> Am 12.02.2015 um 11:06 schrieb Kevin Wolf:
>> >Am 12.02.2015 um 11:02 hat Peter Lieven geschrieben:
>> >>Am 12.02.2015 um 10:58 schrieb Kevin Wolf:
>> >>>Am 12.02.2015 um 10:23 hat Peter Lieven geschrieben:
>> >>>>Am 10.02.2015 um 15:53 schrieb Kevin Wolf:
>> >>>>>Am 10.02.2015 um 15:00 hat Peter Lieven geschrieben:
>> >>>>>>Am 10.02.2015 um 14:54 schrieb Kevin Wolf:
>> >>>>>>>Am 10.02.2015 um 14:42 hat Jeff Cody geschrieben:
>> >>>>>>>>On Tue, Feb 10, 2015 at 02:34:14PM +0100, Kevin Wolf wrote:
>> >>>>>>>>>Am 10.02.2015 um 12:41 hat Peter Lieven geschrieben:
>> >>>>>>>>>>Am 09.02.2015 um 17:09 schrieb Kevin Wolf:
>> >>>>>>>>>>>The CHS calculation as done per the VHD spec imposes a maximum
>> >>>>>>>>>>>image size of ~127 GB. Real VHD images exist that are larger than
>> >>>>>>>>>>>that.
>> >>>>>>>>>>>
>> >>>>>>>>>>>Apparently there are two separate non-standard ways to achieve
>> >>>>>>>>>>>this: You could use more heads than the spec does - this is the
>> >>>>>>>>>>>option that qemu-img create chooses.
>> >>>>>>>>>>>
>> >>>>>>>>>>>However, other images exist where the geometry is set to the
>> >>>>>>>>>>>maximum (65536/16/255), but the actual image size is larger.
>> >>>>>>>>>>>Until now, such images are truncated at 127 GB when opening them
>> >>>>>>>>>>>with qemu.
>> >>>>>>>>>>>
>> >>>>>>>>>>>This patch changes the vpc driver to ignore geometry in this case
>> >>>>>>>>>>>and only trust the size field in the header.
>> >>>>>>>>>>>
>> >>>>>>>>>>>Signed-off-by: Kevin Wolf <address@hidden> ---
>> >>>>>>>>>>>
>> >>>>>>>>>>>Peter, I'm replacing some of your code in the hope that the new
>> >>>>>>>>>>>approach is more generally valid. Of course, I haven't tested if
>> >>>>>>>>>>>your case with disk2vhd is still covered. Could you check this,
>> >>>>>>>>>>>please?
>> >>>>>>>>>>I checked this and found that disk2vhd always sets CHS to 65535ULL
>> >>>>>>>>>>* 16 * 255 independed of the real size.
>> >>>>>>>>>>
>> >>>>>>>>>>But, as the conversion to CHS may have an error its maybe the best
>> >>>>>>>>>>solution to ignore CHS completely and always derive total_sectors
>> >>>>>>>>>>from footer->size unconditionally.
>> >>>>>>>>>>I had a look at what virtualbox does and they only rely on
>> >>>>>>>>>>footer->size. If they alter the size or create an image the write
>> >>>>>>>>>>the new size into the footer and recalculate CHS by the formula
>> >>>>>>>>>>found in the appendix of the original spec.
>> >>>>>>>>>>
>> >>>>>>>>>>Check vhdCreateImage, vhdOpen in
>> >>>>>>>>>>http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Storage/VHD.cpp
>> >>>>>>>>>>
>> >>>>>>>>>>The original spec also says that CHS values purpose is the use in
>> >>>>>>>>>>an ATA controller only.
>> >>>>>>>>>The problem with just using footer->size back then when I
>> >>>>>>>>>implemented this was that from the perspective of a VirtualPC guest
>> >>>>>>>>>run in qemu, the size of its hard disk would change, which you don't
>> >>>>>>>>>want either. Going from VPC to qemu would be ugly, but mostly
>> >>>>>>>>>harmless as the disk only grows. But if you use an image in qemu
>> >>>>>>>>>where the disk looks larger and then go back to VPC which respects
>> >>>>>>>>>geometry, your data may be truncated.
>> >>>>>>>>I believe the vpc "creator" field is different if the image was
>> >>>>>>>>created by Virtual PC, versus created by Hyper-V ("vpc" and "win",
>> >>>>>>>>respectively, I think).  Perhaps we could use that to infer a guest
>> >>>>>>>>image came from VirtualPC, and thus not use footer->size in that
>> >>>>>>>>scenario?
>> >>>>>>>Right, I think we discussed that before. Do you remember the outcome 
>> >>>>>>>of
>> >>>>>>>that discussion? I seem to remember that we had a conclusion, but
>> >>>>>>>apparently it was never actually implemented.
>> >>>>>>>
>> >>>>>>>Would your proposal be to special-case "vpc" to apply the geometry, 
>> >>>>>>>and
>> >>>>>>>everything else (including "win", "d2v" and "qemu") would use the 
>> >>>>>>>footer
>> >>>>>>>field?
>> >>>>>>That sounds reasonable. In any case we have to fix qemu-img create
>> >>>>>>to do not create out of spec geometry for images larger than 127G.
>> >>>>>>It should set the correct footer->size and then calculate the geometry.
>> >>>>>Do I understand correctly that you just volunteered to fix up that whole
>> >>>>>thing? ;-)
>> >>>>I knew that this would happen ;-)
>> >>>>
>> >>>>Regarding the C/H/S calculation. I was just wondering if we should
>> >>>>not set this to maximum (=invalid?) for all newly created images.
>> >>>>That is what disk2vhd does.
>> >>>CHS is what Virtual PC relies on. So I guess if you did that, you
>> >>>would render images unusable by it. Are you sure that disk2vhd does this
>> >>>always? I would have thought that it only does it for large images.
>> >>At least 2.0.1 (latest available version) does this as well as the version
>> >>that I used when I added the hack for d2v creator.
>> >>
>> >>Virtual PC would not be able to use images we create with qemu-img create
>> >>if we use footer->size (which I suppose to reanme to footer->cur_size, btw)
>> >>to calculate bs->total_sectors because we might write data to the end of
>> >>the image which gets truncated in CHS format.
>> >These kinds of problems are why I'd like to keep CHS and size always
>> >consistent when creating an image with qemu-img.
>> Okay, then I would vote for your RFC patch + fixing qemu-img create
>> to not generate out of spec CHS values and just set maximum which
>> then would make vpc_open use footer->size.
> Really the RFC patch or what we discussed above ("vpc" creator = CHS,
> everything else = footer->size)? Once I know what we prefer, I'll send
> the real patch.
> As for heads > 16, that would essentially mean reverting 258d2edb.
> Should be easy to do, the harder part is probably the commit message
> explaining why it's helpful and safe. Note that the commit message of
> 258d2edb claims that it's not out of spec. I _think_ we can do the
> revert with a good explanation, but I'll leave that to you.
> (CCed Charles who wrote that commit)

IIUC, the plan is to revert my old commit and use the footer->size field to
describe images greater than 127 GB.  This change would break other tools
from Virtual PC, Xens vhd-util and maybe others from reading images greater
than 127 GB because the head field would be forced back to using 16 and
these tools won't know to check the footer->size field.  Is there any 
reason not to keep the original commit and still use the footer->size field?

A purist would argue that heads must be 16 for true ATA emulation but allowing
up to 255 doesn't seem to matter and the VHD spec does support up to 2 TB.

- Charles

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