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boot: one hand-off contract, defined once

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Groundwork for the unified loader: before the two paths can share code,
they have to agree on what they are producing, and until now they only
agreed by coincidence.

Every field of the loader/kernel hand-off was #defined in whichever file
happened to need it -- kernel/core/main.c, kernel/dev/vga.c,
kernel/arch/x86_64/e820.c, boot/bios/cdboot.S, boot/bios/stage2main.c and
boot/uefi/main.c, six copies of the same addresses. schboot.h existed for
exactly this and held only the entry magic and the RSDP slot, with a
comment describing the rest of the block it did not define.

The whole block lives there now: the VBE info fields, the verbose and
media flags (with the real-mode segment:offset spelling cdboot.S needs),
the e820 count, map and cap, and the RSDP. Every one of those files
includes it instead of restating it.

This is not a tidy-up. It is the same hazard as defining the entry magic
twice, only quieter -- nothing breaks until one copy moves, and the two
loaders diverging is precisely what produced the EFER.SCE and RSDP bugs,
where a fix on one path silently did not exist on the other.

Verified on both firmwares: BIOS boots to schinit with the same e820,
ACPI and VGA lines as before; UEFI reaches cpu0 with 1018 MB in 7 regions
and its own 1280x800 mode. `make iso` still builds. bootcheck 108 [ok].

Next: the loaders become starters that gather firmware-specific facts and
hand to one common stage. That needs the BIOS side to climb to long mode
first, since common code can only be compiled for one CPU mode -- which
is the risky part, and is why the contract went first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Nexoniarz 2026-08-10 21:37:36 +02:00
commit 0500e4382d
7 changed files with 89 additions and 56 deletions
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11
kernel/arch/x86_64/e820.c
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@ -23,13 +23,10 @@
// than in some new way.
uint64 phystop = PHYSTOP_FALLBACK;
// Where stage2 left the map. Must match E820_COUNT_ADDR / E820_MAP_ADDR in
// boot/bios/stage2main.c, which are themselves relative to VBE_INFO_ADDR.
#define E820_COUNT_ADDR (0x30000 + 0x200)
#define E820_MAP_ADDR (0x30000 + 0x208)
#include "schboot.h" /* the hand-off block */
#define E820_USABLE 1
#define E820_MAXENTRIES MEMREGION_MAX
#define E820_MAXENTRIES SCHBOOT_E820_MAX
// Every usable region, page aligned inwards and clipped to PHYSTOP_MAX.
// kinit1() frees these; see include/memlayout.h for why a list and not just a
@ -59,8 +56,8 @@ struct e820entry {
void
e820_init(void)
{
uint32 count = *(uint32 *)P2V(E820_COUNT_ADDR);
struct e820entry *e = (struct e820entry *)P2V(E820_MAP_ADDR);
uint32 count = *(uint32 *)P2V(SCHBOOT_E820_COUNT);
struct e820entry *e = (struct e820entry *)P2V(SCHBOOT_E820_MAP);
// A count of zero means the firmware declined; anything absurd means we are
// reading something that is not a memory map. Either way the constant is the