Wednesday, October 9, 2013

Scripted boot sequences on OldWorld PowerMacs

Having a decent bootloader on an installed system is a good start, but how do you install it in the first place?

iQUIK relies on having OFW properties or boot arguments to figure out where to look for the configuration file. Having the user pass extra arguments involves knowing those arguments - in our case, the file path and the device path. While the first we know (as a developer), the second depends on the choice of boot media and machine. Of course, we could bake in nasty logic to special case CDs, or scan all partitions on the booted device, or use magic locations in the binary, but none of this is a clean approach.

Ideally we would want to run on a CHRP machine, where we could boot a "CHRP script", which looks something like this:

FreeBSD/powerpc bootloader


boot device:partition,\ppc\kernel;.elf

This would let us verify the target system, and run arbitrary Forth to set up the needed parameters (or even interact with the user). A good example of a complicated CHRP script is the MacOS ROM file on NewWorlds, which is a pretty large blob of Forth followed by an embedded ELF image.

But we're on OldWorld. No bootinfo support. What can we do? We can do pretty much the same, except we have to prefix our Forth with a machine code stub that will call into PROM to interpret the following Forth code. Afterwards, we can pass control to the regular boot loader, which is embedded right after the Forth words. Whereas before the boot partition contained just the iQUIK boot block, now it's a "sandwich" of simply concatenated Forth hand-off stub, the Forth code and the iQUIK boot block. All we have to do is compute the load address correctly, such that the iQUIK boot block falls on its expected linked address.
.section ".start"
.globl _start
.globl _end
.globl _stack_top

 * iQUIK relies on a few variables being set to be able to boot,
 * because it needs to know where to look for the configuration file.
 * This works fine in an installed system, but for install media, these
 * would need to be typed in and be difficult on the user. In a CHRP
 * environment, these could have been set via a bootinfo script, but on
 * OW machines we have no boot-script support.
 * This is where preboot comes in. It's a simple stub that calls OFW
 * to intepret Forth immediately after itself, and then jumps to
 * iQUIK's expected location.
 * Basically, without preboot, the boot partition is just iquik.b.
 * With preboot, the boot partition contains preboot.b + Forth + iquik.b.
 * The memory map looks like this:
 *  PREBOOT_BASE     |
 *  ...              |
 *  preboot.b _start |
 *  preboot.b _end   | forth script
 *  ...              | 
 *  IQUIK_BASE       | iquik.b _start

 * On entry:
 *           r5 contains the PROM entry.
 *           pc is somewhere between PREBOOT_BASE
 *              and IQUIK_BASE.
 mr 22, 5

 * Ensure consistent I+D caches. We could
 * I suppose fetch the cache line size, but this
 * would explode the code size (since we need to
 * do OF calls) and increase complexity at
 * no real gain.
 * Unfortunately have to make it all crufty and
 * hardcode possible addresses used. Can't get
 * our load-addr with the bcl, because the branch&link
 * might fail due to I+D discrepancy. Yikes...
 * Too bad they got rid of 'clcs' in PowerPC.
 addis 10, 0, PREBOOT_BASE@ha
 addi 10, 10, PREBOOT_BASE@l
 addis  11, 0, (IQUIK_BASE + IQUIK_SIZE)@ha
 addi 11, 11,(IQUIK_BASE + IQUIK_SIZE)@l
1: dcbf 0,10
 icbi 0,10
 addi 10,10,4
 cmp 0,10,11
 bne 1b

 * Figure out where we are. r23 contains our load-addr.
 bcl 20, 31, offset
offset: mflr 23

 * Stack.
 addis 1, 23, (_stack_top - offset)@ha
 addi 1, 1, (_stack_top - offset)@l

 * call_prom("interpret", 1, 1, &_end);
 addi 1, 1, -80
 li 4, 1       /* nargs = 1 */
 addis 3, 23, (PROM_INTERPRET - offset)@ha
 addi 3, 3, (PROM_INTERPRET - offset)@l
 stw 3, 0(1)    /* args.service = "interpret" */
 stw 4, 4(1)    /* args.nargs = nargs */
 li 0, 1       /* nret = 1 */
 stw 0, 8(1)    /* args.nret = nret */
 addis 11, 23, (_end - offset)@ha
 addi 11, 11, (_end - offset)@l
 stw 11, 12(1)  /* args.params[0] = &_end */
 mtlr 22
 mr 3, 1
 addi 1, 1, 80
1: mr 5, 22

 * Could try to strlen the forth buffer, but why, we know
 * QUIK is not position-independent.
.section ".rodata"
PROM_INTERPRET: .string "interpret"
Nothing too complicated, but then there's a bunch of hoop-jumping to work around our-not-really-OFW-compatible firmware on certain machines. If it works on OF 1.0.5, it works everywhere ;-). The code also has to be position independent, because the length of the following Forth code is arbitrary, and the base of the iQUIK boot block following that is fixed.

I dub this 'preboot'. It's already checked into the iQUIK git repository, although there are not tools yet to create a new iQUIK boot block containing the preboot-prefixed Forth code. The other real advantage of this approach is that it implements something useful without adding a bunch of hacks and complicating the iQUIK code. On a CHRP system, you would be able to use a CHRP script, and none of this baggage is necessary.

No comments:

Post a Comment