pxeboot and /boot filesystem, share /boot/kernel

I want to tighten up my spaces for diskless machines and I came across
this puzzle with pxeboot:

I can share /usr and most other filesystems, but my individual roots
for the machine each have to have the full kernel.  But /boot/kernel
is rather large these days and totally identical, so I'd rather share
it.

%%

You can only specify a root filesystem location via the dhcp options.
Then, whatever kernel is there in location:/boot/kernel (or rather,
loader.rc) will be booted, which will then pick up the / filesystem
via location:/etc/fstab.

This is not quite what I want, because I want /boot/kernel to be
shared, purely for filesize reasons.  But I can't specify a separate
/boot filesystem.  I can't find an easy way to share /boot but not
share /.

Or in other words, the core of the problem is that I want to share
/boot/kernel but not share /etc/fstab.

%%

So, while writing this mail it occured to me that what I can do is put
a /boot/loader.rc on the individual / filesystems that then redirects
to a common kernel location.  But I don't see how I can make this work
as I do not have the option to point to a new NFS mount in
/boot/loader.conf, or do I?

So what I would want is (on the server):
/diskless-usr/
/diskless-kernel/ [has /boot/kernel/ in it]
/diskless/root1/  [has /boot/loader.conf in it]
/diskless/root2/

DHCP "root-path" is then addr:/diskless/root1

Where
  /diskless/root1/boot/loader.conf 
specifies
  addr:/diskless-kernel/boot/kernel/kernel
instead of just a filename.

Now, the question is how do I make loader.4th able to mount NFS or use
tftp?

%%

I think I have three paths to go here:

1) make pxeboot understand a separate "boot-path" dhcp option.

2) make loader.4th able to use NFS or tftp.  IP is already up by the
   time it is started.

3) only share /boot/kernel/kernel and share a NFS mount for the
   modules, but that's very messy.

Martin
-- 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Martin Cracauer <cracauer@cons.org>   http://www.cons.org/cracauer/
FreeBSD - where you want to go, today.      http://www.freebsd.org/
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