HELP: Cross-compile a kernel?

yassen · Apprentice Joined: 26 Mar 2004 Posts: 194

Hi, I need to install a 64-bit (amd64) system on a machine currently running 32bit Gentoo i386.

This is a server that needs to have a minimal downtime... so I would like to pre-build the 64-bit system in a chroot and then reboot. My 32-bit kernel seems to make problems when trying to chroot into 64-bit environment, however (and that should be expected).

Any general suggestions or specific info what do I need / how to cross-compile a 64-bit kernel? If I do this, will it boot with 32-bit userland?

Thanks a LOT to anyone who would respond.
Yassen
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mdeininger · Posted: Wed Apr 05, 2006 12:58 pm Post subject:

the qemu-docs claim you could emulate an x86_64-type processor... you should thus be able to create the new system inside a virtual machine. can you modify the partition table or would you have a free partition? you could safely install a 64-bit system with qemu on a second partition while your server is live, then just reboot.

either way, I believe the 64-bit kernel should work with the 32-bit userland, just not the other way 'round, thus you can't really chroot into the 64-bit instance unless your kernel supports it. there's a flag in the 64-bit kernels to enable support for 32-bit executables. if you want to cross-compile the kernel, you should be able to do so using the ARCH= option behind the make commands, like

yassen · Apprentice Joined: 26 Mar 2004 Posts: 194

mdeininger: thanks a lot!

Yes, I have a spare partition. It seems that I will need

nixnut · Posted: Wed Apr 05, 2006 6:13 pm Post subject:

Moved from Installing Gentoo to Kernel & Hardware.
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yassen · Apprentice Joined: 26 Mar 2004 Posts: 194

Nope, doesn't work:

yassen · Apprentice Joined: 26 Mar 2004 Posts: 194

Found a nice recent post which claims to do the job but alas, it did NOT work for me:

mdeininger · Posted: Thu Apr 06, 2006 8:11 am Post subject:

hmmm dseng
yeah I ever only tried this with UML, so I'm really not sure about cross-compiling in general. I think the QEMU-approach should really be the easiest -- at least you can play around with that without powering down the server so that should be a big plus

EDIT: unless... could you maybe download an x86_64 kernel for...well... "bootstrapping" of sorts, boot from that, then continue the installation?
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yassen · Apprentice Joined: 26 Mar 2004 Posts: 194

mdeininger: thanks again for caring!

I have a double problem (ohh stupid me!) -- the working sever is actually the UML instance running inside the host where I am trying these things. So I will break it if I replace the host kernel... (unless there are patches for UML that allow 64bit kernel to host an UML instance..)
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mdeininger · Posted: Thu Apr 06, 2006 7:02 pm Post subject:

oh!
lol, okay, that's gettin tricky =)

well, I don't think you need to patch anything on the host-kernel to run a UML kernel, do you?
that is... you do have direct access to the host, aye? well, I'm not quite sure if there is something like a 64-bit UML, so you really could get into jeopardy if you tried to create one... might depend on the toolchain you use to compile the kernel...

is there any way you can move the complete UML to a different host? -- like, freezing it, then copying things over while it's live, as soon as the other is up, running and working, power the original down and let the new one take over? this would allow you to just do a regular re-install, and UMLs should be "easier" to relocate...
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yassen · Apprentice Joined: 26 Mar 2004 Posts: 194