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

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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
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17
boot/uefi/efi.h
View file

@ -171,6 +171,23 @@ typedef struct {
void *ConfigurationTable;
} EFI_SYSTEM_TABLE;
// --- configuration tables -------------------------------------------------
//
// Where UEFI publishes things a BIOS left lying in low memory. The ACPI RSDP is
// the one that matters here: the kernel finds it by scanning the EBDA and
// 0xE0000-0x100000, which is a BIOS convention that firmware is under no
// obligation to honour -- and OVMF does not.
#define EFI_ACPI_20_TABLE_GUID \
{ 0x8868E871, 0xE4F1, 0x11D3, { 0xBC, 0x22, 0x00, 0x80, 0xC7, 0x3C, 0x88, 0x81 } }
#define EFI_ACPI_10_TABLE_GUID \
{ 0xEB9D2D30, 0x2D88, 0x11D3, { 0x9A, 0x16, 0x00, 0x90, 0x27, 0x3F, 0xC1, 0x4D } }
typedef struct {
EFI_GUID VendorGuid;
void *VendorTable;
} EFI_CONFIGURATION_TABLE;
// --- loaded image ---------------------------------------------------------
#define EFI_LOADED_IMAGE_PROTOCOL_GUID \

46
boot/uefi/main.c
View file

@ -323,6 +323,45 @@ efi_type_to_e820(UINT32 t)
// ---------------------------------------------------------------------------
static int
guid_eq(const EFI_GUID *a, const EFI_GUID *b)
{
const UINT8 *p = (const UINT8 *)a, *q = (const UINT8 *)b;
for (UINTN i = 0; i < sizeof(EFI_GUID); i++)
if (p[i] != q[i])
return 0;
return 1;
}
// The ACPI RSDP, from the UEFI configuration table.
//
// The kernel's own search scans 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. The kernel therefore found no RSDP, so no MADT, so no IOAPIC,
// so nothing routed IRQ 4: it booted to a shell prompt on polled serial output
// and then ignored every key, which reads as a broken console rather than a
// missing table.
//
// ACPI 2.0 is preferred over 1.0 because its RSDP carries the XSDT, whose
// entries are 64-bit; the 1.0 RSDT is kept only as a fallback.
static UINT64
find_rsdp(void)
{
EFI_GUID acpi20 = EFI_ACPI_20_TABLE_GUID;
EFI_GUID acpi10 = EFI_ACPI_10_TABLE_GUID;
EFI_CONFIGURATION_TABLE *t =
(EFI_CONFIGURATION_TABLE *)ST->ConfigurationTable;
UINT64 fallback = 0;
for (UINTN i = 0; i < ST->NumberOfTableEntries; i++) {
if (guid_eq(&t[i].VendorGuid, &acpi20))
return (UINT64)(UINTN)t[i].VendorTable;
if (guid_eq(&t[i].VendorGuid, &acpi10))
fallback = (UINT64)(UINTN)t[i].VendorTable;
}
return fallback;
}
EFI_STATUS
EfiMain(EFI_HANDLE image, EFI_SYSTEM_TABLE *systab)
{
@ -341,6 +380,12 @@ EfiMain(EFI_HANDLE image, EFI_SYSTEM_TABLE *systab)
UINT32 fb = setup_framebuffer();
// Read while boot services still exist and the system table is guaranteed
// valid; written to low memory further down, after the firmware has let go
// of it, for the same reason the e820 map is.
UINT64 rsdp = find_rsdp();
print(L"schos: acpi rsdp "); print_hex(rsdp); print(L"\r\n");
// The memory map, and then out. GetMemoryMap must be the last thing before
// ExitBootServices: the key it returns is invalidated by any allocation,
// and AllocatePool for the map buffer is itself an allocation -- hence the
@ -401,6 +446,7 @@ EfiMain(EFI_HANDLE image, EFI_SYSTEM_TABLE *systab)
copy((void *)E820_MAP_ADDR, e820, nent * sizeof e820[0]);
*(UINT32 *)E820_COUNT_ADDR = nent;
*(UINT64 *)SCHBOOT_RSDP_ADDR = rsdp;
// The page tables mboot_entry would have built. entry64low's first act is
// a movabs to 0xffff8000...., so the high half must be mapped before the