Thanks for comments so far.
It looks like the best solution for me is going to be to boot diskless into
a ramdisk. It's been quite a job finding out how to do that; there doesn't
seem to be much in the way of documentation.
Reverse-engineering the source suggests that the following methods exist:
(1) Including an MD image within the kernel itself
options MD_ROOT
options MD_ROOT_SIZE n # kilobytes
then use usr/src/release/write_mfs_in_kernel.c to patch the image into the
kernel. Trouble is you must reserve sufficient space for the image, and
repatch a clean kernel every time you wish to change the ramdisk.
(2) Module preload mechanism
dev/md/md.c calls preload_search_info(mod, MODINFO_NAME)
This accepts a 'module' of type "md_image" or "mfs_root"
It then creates a ramdisk using data at MODINFO_ADDR of MODINFO_SIZE.
These in turn come from sys/kern/subr_module.c which uses a block of
"preload_metadata"
As far as I can tell, either I can put something like
load -t md_image myfile
in boot/loader.rc, or do what release/Makefile does, which is to put
mfsroot_load="YES"
mfsroot_type="mfs_root"
mfsroot_name="/boot/mfsroot"
in boot/loader.conf. I'm not sure if that's sufficient to replace the NFS
root, or if I have to recompile the kernel _without_ options BOOTP and
BOOTP_NFSROOT for this to work, but I'll give it a go later on...
Cheers,
Brian.
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