Re: [Xen-devel] Loading a 32 bit kernel from 64 bit grub-xen

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-07-02 at 09:35 +0100, Jan Beulich wrote:
> >>> On 01.07.14 at 20:27, <psusi@xxxxxxxxxx> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> > 
> > On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> >> Ð Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@xxxxxxxxxx>
> >> ÐÐÑÐÑ:
> >> 
> >>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> >> 
> >>> I have been trying to fix grub to load a 32 bit kernel from the
> >>> 64 bit xen build.  After fixing up one or two minor issues with
> >>> the elf loader, I believe I now have it to the point where it
> >>> jumps correctly to the 32 bit kernel and it crashes there, since
> >>> it is 32 bit code still executing on a cpu in 64 bit mode.  The
> >>> question is how to return the cpu to 32 bit mode *under xen*?
> >> 
> >> 
> >> IIRC it was already discussed not long ago and it seems to be Xen 
> >> limitation. You probably need to ask on xen-devel to be sure.
> >> 
> >> Is it not more simple to use 32 bit grub with 32 bit kernel to
> >> start with?
> > 
> > The problem with that is that you have to know in advance which kernel
> > you are going to boot.  That makes configuring virtual hosts harder;
> > they just want one grub image they can use to chainload whatever the
> > guest wants to install in their domain.
> > 
> > Also there must be a way to do this otherwise a 64 bit kernel running
> > under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
> > Cc xen-devel.
> 
> Perhaps there's some confusion about what "kernel" here means:
> The thing that's the kernel from GrUB's perspective is the hypervisor,
> i.e. xen.gz or xen.efi. While the former expects to be entered in 32-bit
> mode, on the Xen side we all agree that this isn't the way things
> should work under UEFI; Daniel already proposed how to handle this
> case (Xen should be entered in 64-bit mode, with ExitBootServices()
> not having got called yet).
> 
> The thing that you'd traditionally call kernel (e.g. Linux) can be 32-
> or 64-bit irrespective of GrUB's bitness: Xen knows how to load either
> (both for Dom0 and, likely irrelevant here, DomU).

Actually, I read Philips message as refering to launching a domU kernel
from a PV grub-xen, not a dom0 thing.

Ian


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