Ubuntu kernel mystery - using Xen

[Travis Newman]

OK You may have read in other threads that I've been trying to get Xen 
going. It hasn't been easy, and that's in part because of the Ubuntu 
kernel, or in the way the system interacts with the kernel.

Basically, the way Xen works is that Grub loads Xen as the kernel, and 
then the Xen kernel loads the standard vmlinuz type kernel (called 
vmlinuz-xxx-xen0, since it's virtual domain 0). All subsequent virtual 
domains are loaded by the xen daemon through dom0. So...

In order to get it going, you first have to compile the kernel with 
ARCH=xen since you're technically not running i386 anymore. I got the 
kernel to compile, and the xen kernel loaded it just fine, only when I 
booted into Ubuntu, I had no sound, and there was either no support for 
/dev/hdb or there was no support for the fat filesystem.

I have been emailing back and forth with one of the Xen developers, and 
he's sent me a .config to try out, and even posted a kernel he compiled 
for me to download. THEN it occurred to him to try compiling the kernel 
from the vanilla kernel source (no custom patches) without ARCH=xen. And 
sure enough, that didn't work either. I tried compiling a vanilla kernel 
with the .config that ships with the Ubuntu kernel, and this didn't 
work. BUT compiling the Ubuntu kernel tree with either .config worked 
fine. Unfortunately, the Ubuntu kernel patches only affect the 
architectures it supports (i386, ppc, AMD64, etc), so using the Ubuntu 
kernel tree to build a Xen kernel doesn't work.

I even installed Fedora and used their rpms, and got Ubuntu to boot with 
the Fedora Xen kernel, and it acted exactly the same-- no hdb1 
partition, and no sound-- though they both worked perfectly fine in Fedora.

SO. There's something disturbing about the way Ubuntu interacts with the 
kernel, since custom kernels not built off of the Ubuntu kernel tree 
don't work with certain things.

Does anyone know what might be causing this, how to get around it, or 
how to fix it?

(crossposted in an attempt to get the most input)