[TUHS] kernel boots kernel in 1977

[Dan Cross]

On Wed, Sep 18, 2024 at 8:05 PM Bakul Shah via TUHS <tuhs at tuhs.org> wrote:
> Can you not avoid resetting the machine? This can be treated almost as sleep in the old kernel, wakeup in the new one! You do have to reset devices individually (which may not always work if it requires assistance from some undocumented firmware).

Perhaps this is what you mean when you mention assistance from
firmware, Bakul, but it may be useful to consider that _many_ devices
are touched by e.g. a BIOS or UEFI or whatever well before the OS is
even loaded.

If one steps back and considers the utility of a BIOS/UEFI (and I
often lump these into the same category), there are three principal
reasons for it: 1) back in the bad old days, we could offload common
IO functions into code stored on a ROM, freeing up precious RAM for
programs. 2) firmware provides a layer of indirection between the
system and the host software, allowing both to vary while continuing
to work with newer versions of the other. And finally 3) firmware
facilitates bootstrapping the system by providing the host some way to
access devices and locate and load an OS image, er, before the OS
image is loaded. SOMETHING has to get enough code loaded from
somewhere to start the system; often times that's firmware.

Anyway, the last two suggest that device state can be arbitrarily
munged before the OS takes over, and an actual reset at the device
level might wipe out some state the OS depends on. Consider, for
example, programming PCI BARs; on a "modern" x86-64 system with UEFI,
this is done by firmware in the PEI layer, and the OS may expect that
to already be set up by the time it is probing buses. An actual
honest-to-goodness reset will probably wipe the BARs, requiring the
host OS to program them (ironically, many OSes are already equipped to
do so, as they have to handle these cases for e.g. PCI hotplug events,
though many don't do it in the "ordinary" discovery and initialization
phase of boot).

I suppose the point is that a reset is great because it really does
wipe out state, but it may also be a bummer because, well, it really
does wipe out state. :-)

        - Dan C.

> On Sep 18, 2024, at 4:58 PM, ron minnich <rminnich at gmail.com> wrote:
>
> well, yes, on many systems, there's a lot that runs before the kernel. But if you have a risc-v system with oreboot, you own the system. The problem is that on most of these systems a reset will stop the dram clock for a little bit, or glitch clock enable, or dram power, or whatever. New systems are not designed to allow this.
>
> Ideally, we could force a reset of everything save memory, but modern systems are not designed in this way. Most annoying.
>
> On Wed, Sep 18, 2024 at 4:38 PM Bakul Shah <bakul at iitbombay.org> wrote:
>>
>> I would prefer old kernel to new kernel handoff if it can be made to work reliably. Nowadays there are a lot of things that run before the kernel gets control.
>>
>> On Sep 18, 2024, at 3:38 PM, ron minnich <rminnich at gmail.com> wrote:
>>
>> Interesting about the amiga. I'm assuming their firmware zeros memory on reset, so you have to do handoff from kernel to kernel, not via a reset and so on?
>>
>> What was particularly nice about the V6/PDP-11 case: we were able to yank reset, which let us cleanly reset/disable devices, because everything was in memory when we got back. I miss the simplicity of the old machines.
>>
>> On Wed, Sep 18, 2024 at 3:07 PM Christian Hopps <chopps at chopps.org> wrote:
>>>
>>>
>>> We had/have this functionality in the Amiga port of NetBSD.
>>>
>>> It is implemented as `/dev/reload` device and you copy a kernel image to it. In locore.s there's code that copies the kernel image over top of the running kernel and then restarts. I believe for it to work nothing below the copy code in locore.s can change :)
>>>
>>> Thanks,
>>> Chris.
>>>
>>> Phil Budne <phil at ultimate.com> writes:
>>>
>>> > ron minnich wrote:
>>> >> But I'm wondering: is Ed's work in 1977 the first "kernel boots kernel" or
>>> >> was there something before?
>>> >
>>> > There was!  The PDP-7 UNIX listings contain a program trysys.s
>>> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/sys/trysys.s
>>> > that reboots the system by reading a.out into user memory (in the high
>>> > 4K of core), then copies it to low memory and jumping to the entry
>>> > point.  The name suggests its original intended use was to test a new
>>> > system (kernel).
>>> >
>>> > P.S.
>>> > Normal bootable system images seem to have been stored in reserved
>>> > tracks of the (fixed head) disk (that are inacessible via system calls):
>>> >
>>> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/sys/maksys.s
>>> > reads a.out and uses I/O instructions to write it out.
>>> >
>>> > P.P.S.
>>> > Accordingly, I put together a "paper tape" for booting the system:
>>> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/other/pbboot.s
>>> >
>>> > P.P.P.S.
>>> > The system (kernel) is 3K words, the last 1K of low memory
>>> > used for the character table for the vector graphics controller.
>>> >
>>> > The definitions for the table are compiled by
>>> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/cmd/cas.s
>>> > from definition file
>>> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/sys/cas.in
>>> > (after, ISTR, figuring out the ordering of the listing pages!)
>>> >
>>> > I don't think we ever figured out how the initial character table
>>> > is loaded into core.  One thing that was missing from the table
>>> > was the dispatch array, which I recreated:
>>> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/other/chrtbl.s
>>> >
>>> > The system (kernel) could be built for a "cold start", reloading the
>>> > disk (prone to head crashes?) from paper tape? But I don't think
>>> > anyone ever reconstructed the procedure for rebuilding a disk that way.
>>> >
>>> > The disk was two sided, and the running system only used one side:
>>> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/cmd/dsksav.s
>>> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/cmd/dskres.s
>>> > appear to be programs to save and restore the filesystem from the
>>> > "other" side of the disk.
>>>
>>
>