nexoniarz/SchOS
nexoniarz/SchOS
Watch
1
0
Fork
You've already forked SchOS
 0
Code Issues Pull requests Projects Releases 1 Packages Wiki Activity Actions

acpi: take the RSDP from the loader when firmware gives it to us

Browse source 
find_rsdp() scanned the EBDA and 0xE0000-0x100000. That is a BIOS
convention, and UEFI firmware is under no obligation to leave a copy
there -- OVMF does not. So the kernel found no RSDP, no MADT and no
IOAPIC, and nothing routed IRQ 4. Serial output is polled, so the machine
booted all the way to a shell prompt and then ignored every key, which
reads as a broken console rather than a missing table.

The UEFI loader reads the RSDP out of the EFI configuration table,
preferring ACPI 2.0 for its XSDT, and leaves it at SCHBOOT_RSDP_ADDR --
the same low-memory hand-off block the e820 map already uses, now
described in include/schboot.h so the two sides cannot disagree.

The kernel validates it rather than trusting it: the BIOS path never
writes that slot, so there it holds whatever was in memory, and following
a wild pointer faults before there is anything to report the fault with.
Signature and checksum do that. It deliberately does not require 16-byte
alignment -- that rule is about searching the BIOS area, and OVMF's RSDP
sits at 0x3f77e014, so demanding it threw away the one address this code
exists to use.

UEFI now reports "acpi: 1 cpu, lapic at ..., ioapic id 0" and the ISO
boots to an interactive shell: uname, ls /bin (241), writing and reading
a file, ps, /proc/schfs -- all driven over serial.

BIOS unchanged: bootcheck 108 [ok], same ACPI line as before.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Nexoniarz 2026-08-09 22:02:57 +02:00
commit 9735279970
4 changed files with 101 additions and 0 deletions
Download patch file Download diff file Expand all files Collapse all files

15
include/schboot.h
View file

@ -23,6 +23,21 @@
#define SCH64_MAGIC0 0x36484353 /* "SCH6" */
#define SCH64_MAGIC1 0x4f4f4234 /* "4BOO" */
// Where the loader leaves the ACPI RSDP, as a little-endian 64-bit physical
// address. Zero means "not provided, go and look for it yourself".
//
// The kernel's own search scans the EBDA and 0xE0000-0x100000, which is a BIOS
// convention. UEFI publishes the RSDP in its configuration table instead and is
// under no obligation to leave a copy anywhere in particular; OVMF does not.
// Without it the kernel finds no MADT, so no IOAPIC, so no interrupt routing --
// the machine boots to a prompt on polled serial output and then ignores every
// key, which looks like a broken console rather than a missing table.
//
// 0x30000 is the same low-memory hand-off block the BIOS loader uses:
// +0x100 verbose flag, +0x200 e820 count, +0x208 the e820 map (32 entries of
// 24 bytes, ending at 0x508). 0x600 is clear of all of it.
#define SCHBOOT_RSDP_ADDR (0x30000 + 0x600)
#ifndef __ASSEMBLER__
struct sch64_header {