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

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

boot/uefi/main.c

@@ -594,7 +594,7 @@ place_staged(void)
 }
 
 // Load PT_LOAD segments to p_paddr. Physical addresses, not virtual: the
-// kernel is linked at 0xffff800000100000 but must be *placed* at 0x100000, and
+// kernel is linked at 0xffff800001000000 but must be *placed* at 0x1000000, and
 // p_paddr is where the linker script says each segment belongs.
 static UINT64
 load_elf(EFI_FILE_PROTOCOL *file)

include/arch/memlayout.h

@@ -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) {

include/arch/schboot.h

@@ -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

kernel/kernel.ld

@@ -7,13 +7,13 @@ OUTPUT_FORMAT("elf64-x86-64")
 OUTPUT_ARCH(i386:x86-64)
 ENTRY(_start)
 
-mboot_load_addr = 0x00100000;
+mboot_load_addr = 0x01000000;   /* KERNPHYS in include/arch/memlayout.h */
 
 SECTIONS
 {
   /* Link the kernel at this address: "." means the current address */
   /* Must be equal to KERNLINK */
-  . = 0xFFFF800000100000;
+  . = 0xFFFF800001000000;
 
   PROVIDE(begin = .);