Subject: Two NetBSDs on one (i386) drive
To: None <>
From: Robert Elz <kre@munnari.OZ.AU>
List: tech-kern
Date: 12/19/2001 19:45:05
I'd like to be able to have two distinct NetBSD setups on one i386

At the minute it isn't possible (well, not cleanly) because both /boot
and the kernel find the root by searching the partition table for a
NetBSD partition, and assuming that the first they find is it.

I think an easy way to adjust this (easy for someone who understands
i386 instructions anyway, which isn't me), is for the search to start
at N, and do N%4 (N+1)%4 (N+2)%4 and (N+3)%4  - when N is 0, that's
exactly what happens now, when it isn't, then it will allow a different
NetBSD root to be found by setting N to be something different than 0.

Then, we can take N to be something passed in from the lower level boot
code (mbr -> 1st stage boot -> 2nd stage boot -> kernel).

This is safe, even with current boot code that does none of this, as
currently we assume only one NetBSD partition (slice in other people's
terminology) per drive.   That is, where the search starts makes no
difference, except to how many iterations around the loop it takes
(which is bounded at 4 anyway...).   Of course, it would mean that
putting two NetBSD partitions on a drive would be a poor idea if there
were no boot blocks to support it.

After that, all that's left is for the mbr code to leave the partition
number it is booting somewhere for the later boot stages to find.  I know
space there is very tight (especially in the boot selector version), but
fortunately, very very little is going to need to be added, as the mbr code
is already having to find the partition to boot from - all it needs to do
is save that - we could use almost any value that is good for the mbr code
as the "which partition" flag, it doesn't have to be 0 1 2 3 at that stage.

If there is prior art, from some other boot system, in how to pass this
info through, then it would be good to copy it of course - that way people
who use that other boot system instead of NetBSD's mbr code would win as well.

Note all of this is a fairly safe and painless change - for anything to
actually alter from the current state of the world, three things would
have to happen - the low level boot code would have to save the boot
partition somewhere (or that somewhere would have to accidentally be set
to a suitable value .. which could happen of course) - the upper level boot
code & kernel would have to be updated to use the value passed in (after
testing it for sanity of course), and there would have to actually be 2
NetBSD partitions on the drive.


ps: I know the other way - of making a kernel & /boot combination that
look for a value other than 169, and using that as the alternate boot.
That's the "unclean" method, as it means the system using them can't just
be copied to the "normal" NetBSD place and used, which is what I'd like to
be able to do.