Re: [Xen-devel] guest kexec / domU boot loader

To:	"Gerd Hoffmann" <kraxel@xxxxxxx>
Subject:	Re: [Xen-devel] guest kexec / domU boot loader
From:	"Jacob Gorm Hansen" <jacobg@xxxxxxx>
Date:	Fri, 10 Mar 2006 15:11:29 +0100
Cc:	Xen devel list <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 3/10/06, Gerd Hoffmann <kraxel@xxxxxxx> wrote:
>   Hi,
>
> I have some more few question/problems wrt. kexec.  They are not
> strictly kexec-related, I think any complete control pass-over from one
> kernel to another has them, i.e. if someone wants to combine mini-os and
> grub to build a boot loader the same issues will come up:

> Problem number one are the different "magic" pages: start info, shared
> info, xenstore, console.  Right now they are allocated at a random place
> in guest physical memory (depending on kernel and initrd size).  One
> problem with that is that they are in the area where you might want to
> load a kernel to.  Sure you can map them somewhere else in virtual
> space, but if you want to keep things very simple for a mini-kernel or
> bootloader and create a simple virt==phys identity-mapping it's a nasty
> special case.

I may be repeating myself here, but if you have a look at the file
linux_loader.c in extras/cstrap, in my tree at
http://www.distlab.dk/hg/index.cgi/xen-gfx.hg , you should see a
solution to the problem of these special pages. Since the domU
bootloader sets up the virtual space of the guest (code mostly pasted
from xc_linux_build.c), pointing the relevant virtual addresses to the
correct mfns (from the start info page) by overriding entries in the
page_array[] is fairly trivial.

Best regards,
Jacob

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