Re: [Qemu-devel] Add support for new image type

[Stefan Weil <weil@mail.berlios.de>]

Am 01.03.2012 21:10, schrieb Stefan Weil:
> Am 01.03.2012 20:45, schrieb Kai Meyer:
>> On 03/01/2012 08:03 AM, Kevin Wolf wrote:
>>> Am 29.02.2012 22:52, schrieb Kai Meyer:
>>>> Is it possible to extend qemu to support a new image type? I have an
>>>> image type that is ready for consumption and I'm looking for the
>>>> integration point between qemu and the new image format.
>>> Which image format do you want to get integrated?
>>>
>>> Have a look at block/qcow2.c to get an idea of what a qemu block driver
>>> looks like. At the bottom of the file there is a struct that contains
>>> function pointers to all exported functions, so this is usually a good
>>> place to start exploring a driver.
>>>
>>> Kevin
>>
>> Great, this is exactly what we're after. I work for StorageCraft, and 
>> we would like to figure out some way to allow qemu to directly 
>> consume our image-based backups. It would provide us with user-space 
>> mounting (via libguestfs) as well as booting VMs directly from Backup 
>> images. We already have a proprietary image access library that 
>> provides block-wise access to our image files. I have been able to 
>> scratch together a proof of concept already, which I am really 
>> pleased with.
>>
>> I see only two roadblocks for which I don't have immediate answers for.
>>
>> 1) Licensing
>> Is it possible to license our contributions in such a way that we do 
>> not need to open the source code of our image access library?
>
> This is not possible if you want to link your image access library 
> statically.
>
> QEMU uses GPL v2. You can use a patched QEMU internally,
> but as soon as you want to give it to customers (or get it integrated
> in the official source tree), you must publish all code which is needed
> under an open license.
>
> If your image access library is a shared library (linked at runtime),
> the situation is more difficult. The intention of the GPL is still
> that you have to publish your code (this is also what the FSF says),
> but there are different opinions for GPL v2. See
> http://en.wikipedia.org/wiki/GNU_General_Public_License for
> more information.
>
> I personally don't like the idea of QEMU supporting closed source
> shared libraries.
>
>> 2) External dependency on our image access library.
>> We do not want to force qemu to require our image access library to 
>> be present to build. Would it be better to do a conditional build 
>> (./configure --with-spf) or a run-time check for our image access 
>> library?
>
> You'll need the conditional build for those people who don't want support
> for your image type.
>
> Regards,
>
> Stefan Weil


I forgot to mention a possible solution: instead of using your image 
access library
(which is closed source), you could write new access code (maybe with 
reduced
functionality) and publish that code under an open source license.

QEMU uses this approach for some image types where we don't have the
original source code (for example block/vpc.c or block/vmdk.c).

Regards,
Stefan Weil