[gentoo-user] Re: 64-bit kernel on a 32-bit installation

[ABCD <en.ABCD@gmail.com>]

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Mark Knecht wrote:
> Hi,
>    I lurk on the LKML, say hi once in awhile, ask a question once in
> awhile, and try to read at least the interesting to a non-programmer
> posts. I was curious about this one that came up today. Seems like
> this is a natural for Gentoo.
> 
>    I have a Gentoo 64-bit setup but have had lots of troubles over the
> years (far less now though) with web media and other things that need
> to be more Windows compatible. (I do audio work with my Gentoo boxes -
> interface to studios and a few bands, etc) I've found that my 32-bit
> Gentoo installations have been more compatible than 64-bit. Outside
> stuff like Java is better. In general when I have a problem I wonder
> if it's because I'm running 64-bit.
> 
>    How would one go about building a 64-bit kernel on a 32-bit machine
> with Gentoo? I presume that's mostly just how I configure the kernel,
> along with maybe some cross-compile options? Are there any projects
> going on in this area where I might become a test case? Wiki? Docs?
> 
>    Do others see value - getting 64-bit memory management, new CPU
> flags, etc., but keeping the apps 32-bit for compatibility?
> 
> Take care,
> Mark

Personally, I am using a 64-bit kernel with a 32-bit userland.  My setup
is a bit more complicated than the usual, because I have a
x86_64-pc-linux-gnu-gcc that will build 32-bit as well as 64-bit
binaries.  The simpler version of what I use is:

# emerge crossdev
# crossdev -t x86_64-pc-linux-gnu

Then, you can use something like the following to actually build a
64-bit kernel (personally, I always use out-of-tree builds, and create a
GNUmakefile that calls the Makefile in the current directory with all
the options I want):

(in the kernel build directory)
# make -C /path/to/sources O=`pwd` ARCH=x86 \
    CROSS_COMPILE=x86_64-pc-linux-gnu- \
    menuconfig

I have found that just about everything works perfectly in my 64-bit
kernel with 32-bit userland, *except* VirtualBox, which I have to run
the 64-bit version of from a chroot.  I also personally handle all
external kernel modules, and add them to package.provided when
necessary, so portage doesn't have to think about them.

PS:
I was going to outline all the patches, etc. that I needed for a
multilib gcc/glibc, but then realized that you probably didn't need that
much detail.

- --
ABCD
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