Easiest way to build 64bit kernel on 32bit system?

[bbee]

I'd like to dump my old power hungry Athlon64 desktop system and replace it with a thin client of sorts, so I'm working on moving all my apps from the desktop to my HTPC (with the intention of using them remotely with ssh + freenx or xvnc).
The desktop is running 64bit kernel/userland, has 4GB of memory and usually pages out a few GB to disk. The HTPC has a core2 cpu and is running 32bit userland/kernel; it will take max. 3GB of memory so memory pressure will be even higher there once I migrate the apps from the desktop. I want to keep the userland 32bit to ease some of the memory pressure, but I'd like to run a 64bit kernel to avoid stuff like highmem and to keep the ability to run 64bit KVM guests.

So my question is: Is there a "gentoo way" of producing a toolchain for a 64bit target on an otherwise 32bit system? And a way of tricking the kernel sources installed by portage to /usr/src into using it? And a way of maintaining/updating the toolchain with the regular tools (emerge) of the 32bit system?
Short of a 64bit chroot ofcourse, since that would duplicate a million utilities incl. portage itself, just to build a kernel.. I don't want a parallel install of python.. I suppose I could use buildroot to do it, but I'd like a method that fits cleanly into gentoo if possible.

Apologies if this has been answered before, I couldn't find anything relevant with the search.

[i92guboj]

I think that you don't need anything special, since the kernel doesn't link to anything outside it, all you need is a compiler capable to produce x86_64 code, and gcc can regardless of the architecture it was compiled for.

Try this:

[bbee]

[Hu]

An i686 gcc cannot create x86_64 code. I think i92guboj meant that your gcc could execute on any host architecture, and would be satisfactory if it produced x86_64 output code. You can build a gcc that runs on i686-pc-linux-gnu, but produces code for x86_64-pc-linux-gnu. Such a gcc could then execute on your 32-bit userland to build a 64-bit kernel, once you find the right options for making the kernel build process invoke it. I recall the Makefile variable CROSS_COMPILE being involved, but I have not done kernel cross compilation recently enough to provide more details.

You may find it useful to add V=1 to the make call, so that you can see the commands it executes. That should make it easier to see when you find the right set of arguments.

[bbee]

After I posted I googled the error message, and I thought that i92guboj might have been referring to a multilib system, which isn't enabled by default on x86. (I'm not even sure if it's supported on x86)
Anyway I remembered that sys-devel/crossdev exists, and after 30 minutes and:

[Captain Newbie]