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kernel: load at 16 MB, clear of what firmware keeps low

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From 1 MB the 34 MB kernel ran across 8 MB, where OVMF keeps ACPI NVS
memory that outlives the firmware -- so it was overwritten on every
boot, after ExitBootServices. Firmware leaves its leftovers low; above
16 MB is where a machine is most reliably empty.

KERNPHYS in memlayout.h, and kernel.ld agreeing with it. Nothing else
named the old address. Under OVMF the range is now only boot-services
memory: staged, copied home after the firmware exits, no warning. Boots
as a CD and as a USB disk, 1 and 4 CPUs.

The 15 MB below the kernel is not given to the page allocator yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
This commit is contained in:
Nexoniarz 2026-10-24 00:13:53 +02:00
commit 5ed8f1b0d5
4 changed files with 14 additions and 5 deletions
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11
include/arch/memlayout.h
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@ -80,7 +80,16 @@ extern uint64 lowtop;
// Key addresses for address space layout
#define KERNBASE 0xFFFF800000000000 // First kernel virtual address
#define KERNLINK (KERNBASE+EXTMEM) // Address where kernel is linked
// Where the kernel is placed in physical memory, and linked above KERNBASE.
//
// 16 MB, not the 1 MB xv6 used. The kernel is 34 MB with the RAM disk in it,
// and from 1 MB that ran across 8 MB, where OVMF keeps ACPI NVS memory --
// which outlives the firmware, so the kernel was copied over it after
// ExitBootServices on every boot. Firmware keeps its leftovers low; above
// 16 MB is where a machine is most reliably empty. kernel/kernel.ld repeats
// this number and has to agree.
#define KERNPHYS 0x1000000
#define KERNLINK (KERNBASE+KERNPHYS) // Address where kernel is linked
#ifndef __ASSEMBLER__
static inline addr_t v2p(void *a) {

2
include/arch/schboot.h
View file

@ -34,7 +34,7 @@
//
// 0x30000 is clear of stage2 (loaded at 0x7E00 and well under 0xA000), the
// multiboot kernel-search buffer at 0x10000-0x14000, and the kernel's own load
// address at 0x100000.
// address at KERNPHYS, 16 MB.
//
// Both loaders write it, so a field's meaning must not depend on which one did.
// Where they genuinely differ -- the BIOS path knows what medium it booted