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RE: [Xen-devel] RFC/Patch: Support for other bootloaders

To: "Michal Ostrowski" <mostrows@xxxxxxxxxxxxxx>, "Tim Deegan" <Tim.Deegan@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] RFC/Patch: Support for other bootloaders
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Wed, 23 Mar 2005 17:30:12 -0000
Cc: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxxx>, <ian.pratt@xxxxxxxxxxxx>, <ian.pratt@xxxxxxxxxxxx>
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Thread-topic: [Xen-devel] RFC/Patch: Support for other bootloaders
> > It should be.  xen-vmlinux is an ELF, so if we can be 
> *sure* that the
> > largest file offset in an ELF header is the same as the size of the
> > file, then it would be possible to just do
> > 
> >  $ cat vmlinux-xen0 initrd-xen0 >pseudo-initrd
> > 
> >  boot: xen pseudo-initrd.
> > 
> > and then have Xen root around in the ELF headers to see 
> where the break
> > is.
> > 
> Sure, you could do that.  You could also define, in Linux, two symbols
> "initrd_start", "initrd_end".
> Then, use objcopy to insert a ramdisk in between the two.  It's not
> "cat", but you can just create a wrapper shell script around the
> objcopy.
> Linux then checks to see if the two symbols point to the same spot; if
> they don't it's got a ramdisk embedded in it.

You mentioned that ppc Linux already has a build target for making an
initrd and wrapping it with a kernel. Do other architectures have this?
How does it work? What are the prospects of getting such funcitonality
added to native i386? Can we ease make the procedure for producing a
wrapped file easy when your building on a different machine from the
target (i.e. where mkinitrd needs different args)?
> > Suporting gzipped vmlinux should be possible too; Xen needs 
> to unzip the
> > kernel and then knows that the next byte after the gzipped 
> input will be
> > ramdisk.  (As long as the bootloader doesn't helfully unzip the
> > 'ramdisk' for us.)

As I recall, we currently rely on having GRUB uncompressing the kernel
and initrd. It's a bit grim to do this in Xen, but doable.

> Sure,  but I thought the direction we're going in is to use 
> your wrapper
> around the Xen binary.  If that's the case then I don't see 
> much further use for my code.  

I don't think we have all the inputs to have a firm direction yet. 


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