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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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16
boot/uefi/main.c
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@ -27,13 +27,9 @@
#include "efi.h"
#include "../../include/schboot.h"
// Where the kernel expects to find things. These are the BIOS loader's
// addresses and they are deliberately unchanged: the kernel should not be able
// to tell which firmware booted it.
#define VBE_INFO_ADDR 0x30000
#define E820_COUNT_ADDR (VBE_INFO_ADDR + 0x200)
#define E820_MAP_ADDR (VBE_INFO_ADDR + 0x208)
#define E820_MAXENTRIES 32
// Where the kernel expects to find things comes from schboot.h, included
// above: the hand-off is one contract and both loaders write the same one.
#define E820_MAXENTRIES SCHBOOT_E820_MAX
#define E820_USABLE 1
#define E820_RESERVED 2
#define E820_ACPI 3
@ -438,7 +434,7 @@ setup_framebuffer(void)
// block as the BIOS path leaves it, and the kernel takes exactly four
// fields out of it -- so those four are what get written, at the offsets
// it uses.
UINT8 *vbe = (UINT8 *)VBE_INFO_ADDR;
UINT8 *vbe = (UINT8 *)SCHBOOT_VBE_INFO;
zero(vbe, 512);
UINT32 bpp = 32; // GOP's 8-bit formats are 32bpp
@ -612,8 +608,8 @@ EfiMain(EFI_HANDLE image, EFI_SYSTEM_TABLE *systab)
__asm__ volatile("cli");
copy((void *)E820_MAP_ADDR, e820, nent * sizeof e820[0]);
*(UINT32 *)E820_COUNT_ADDR = nent;
copy((void *)SCHBOOT_E820_MAP, e820, nent * sizeof e820[0]);
*(UINT32 *)SCHBOOT_E820_COUNT = nent;
*(UINT64 *)SCHBOOT_RSDP_ADDR = rsdp;
// The page tables mboot_entry would have built. entry64low's first act is