Re: [kvm-devel] Making qemu images executable (and store command line arguments in them =P)

[Avi Kivity <avi@qumranet.com>]

Mark Williamson wrote:
>> I've been giving some thought to Anthony's idea:
>>
>> http://kvm.qumranet.com/kvmwiki/Specs/StoringCommandLineInImage
>>
>> However, maybe I'm just too much on vacations, but I don't seem to
>> come up with a nice way of doing this. Everything keeps coming back to
>> creating a new 'container' image format and then implementing block
>> layer functions that only add the number of sectors occupied by the
>> command-line to the read and write calls made by QEMU, and then just
>> relay those calls to the image-specific functions. That doesn't sound
>> very efficient.
>>     
>
> It's not necessarily that pretty, but I wouldn't have thought that adding a 
> simple offset to block operations will have a measurable performance impact 
> (given the latencies involved in block accesses anyhow, and the amount of 
> data transferred each time).
>
>   

Right.

>> The '#!' trick works nice with scripts, but I don't see it playing
>> very well with images. ¿Comments? ¿Pointers?
>>     
>
> Well, it's not really necessary, but it would be darn cool :-)  Another cool 
> (but admittedly twisted - get the brain soap ready!) thing to do would be to 
> statically link a qemu, and then include a virtual machine config and disks 
> in a section of the elf file (inspired by glick: 
> http://blogs.gnome.org/alexl/2007/08/07/experiments-with-runtime-less-app-bundles/).  
> So then you'd have an "executable" VM image which doesn't need a Qemu runtime 
> to be available.  There are various variations you could do on this basic 
> premise in order to make the file you carry around less terrifyingly huge!
>   

That would make the VM not transportable (think moving from an x86_64
host to an i386 host) and would tie it to a particular version of kvm
userspace.

> Anyhow, enough of my random ideas...  I was thinking about container formats.
>
> I've missed some of the discussion, but wouldn't tar be an obvious choice?  It 
> can expand easily out to a directory hierarchy containing config file and 
> multiple virtual disk files, there are standard tools that can manipulate it 
> and standard libraries that can be used by Qemu in order to get at the 
> contents.  Only problem I see with this approach is that sparse file handling 
> might get a bit strange (using real sparse files vs using tar's 
> represesntation of sparse files vs compatibility with tars that don't support 
> them!).
>   

Also it can't be used in-place, like Anthony's header or the
metadata-in-snapshot idea.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.