M2 step 4: relink the kernel into the higher half

The kernel now links at 0xFFFF_FFFF_8000_0000 (code_model .kernel) and
loads low via the linker script's AT() clauses (.text at 1 MiB). A new
asm _start installs a 64 KiB kernel-owned .bss stack — the loader stack
is a low address that goes away with the identity map — and calls the
Zig entry, which reaches boot_info through the physmap. The user-pf test
blob reads the LAPIC through the physmap window (still present|user, ec
0x5). The low identity map still coexists in the kernel's tables as the
safety net. Suite 27/27.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/kernel/arch/x86_64/isr.s

@@ -10,6 +10,28 @@
 
 .text
 
+# _start: the kernel entry. The loader jumps here (higher-half address) with
+# boot_info in RDI, still on the loader's low stack. Switch to a kernel-owned
+# stack in .bss (the loader stack is a low address that goes away once the low
+# half is dropped), keeping RDI, then call the Zig entry. kmainEntry never
+# returns; the hlt loop is a belt-and-braces backstop.
+.global _start
+_start:
+    leaq bootstrap_stack_top(%rip), %rsp
+    call kmainEntry
+1:  hlt
+    jmp 1b
+
+# The kernel's initial stack (used until the scheduler hands each task its own).
+# 64 KiB: kmain's discovery path includes the recursive AML interpreter, so it
+# needs more than a token stack. Lives in .bss (zeroed, higher-half).
+.section .bss
+.balign 16
+bootstrap_stack:
+    .skip 65536
+bootstrap_stack_top:
+.text
+
 # gdt_flush(rdi = *GDT descriptor): load the GDT, reload the data segment
 # registers to the data selector, and reload CS to the code selector. CS can't be
 # set with mov, so we far-return through the caller's own return address.
@@ -152,14 +174,14 @@ user_prog_start:
 user_prog_end:
 
 # The isolation-proof program: read a kernel-only page from ring 3. The LAPIC
-# page is unconditionally mapped supervisor (paging.zig), so this must take a
-# #PF with error code 0x5 (present | user) before any access happens. The
-# address goes through %ecx (zero-extending) — an absolute `mov 0xFEE00000,%rax`
-# would sign-extend the disp32 to 0xFFFFFFFF_FEE00000 and miss the page.
+# lives in the kernel's physmap (physmap_base + 0xFEE00000) as a supervisor
+# page, so this must take a #PF with error code 0x5 (present | user) before any
+# access happens. movabs loads the full 64-bit higher-half address (a disp32
+# would sign-extend and miss).
 .global user_pf_start
 .global user_pf_end
 user_pf_start:
-    mov $0xFEE00000, %ecx
+    movabs $0xFFFF8800FEE00000, %rcx
     mov (%rcx), %rax
 1:  jmp 1b
 user_pf_end:

src/kernel/arch/x86_64/linker.ld

@@ -1,12 +1,18 @@
-/* Kernel link layout.
+/* Kernel link layout — higher half.
  *
- * The kernel is linked at a fixed low physical address (set by `image_base` in
- * build.zig). UEFI runs with memory identity-mapped, so the bootloader can load
- * each PT_LOAD segment to the physical address matching its virtual address and
- * jump straight to _start — no page tables to build yet. (Moving to a
- * higher-half virtual base is a later step, once the bootloader sets up paging.)
+ * The kernel is linked to *run* in the higher half (virtual base
+ * 0xFFFF_FFFF_8000_0000, matching danos.kernel_virt_base and build.zig's
+ * image_base) but is *loaded* low. Each section's load address (LMA) is its
+ * virtual address minus KERNEL_VIRT_BASE via AT(), so the ELF's p_paddr lands
+ * at a low physical address (.text at 1 MiB) that the loader can allocate and
+ * copy into. The loader maps p_vaddr (high) -> p_paddr (low) in its bootstrap
+ * tables and jumps to the high entry; the kernel then builds its own tables
+ * with the physmap and abandons the identity map. Requires LLD (build.zig pins
+ * it) — the self-hosted linker ignores PHDRS/AT()/section order.
  */
 
+KERNEL_VIRT_BASE = 0xFFFFFFFF80000000;
+
 ENTRY(_start)
 
 /* One loadable segment per permission set, so the loader can map .text as R+X,
@@ -18,23 +24,22 @@ PHDRS {
 }
 
 SECTIONS {
-    .text ALIGN(4K) : {
+    .text ALIGN(4K) : AT(ADDR(.text) - KERNEL_VIRT_BASE) {
         *(.text .text.*)
     } :text
 
-    .rodata ALIGN(4K) : {
+    .rodata ALIGN(4K) : AT(ADDR(.rodata) - KERNEL_VIRT_BASE) {
         *(.rodata .rodata.*)
     } :rodata
 
-    .data ALIGN(4K) : {
+    .data ALIGN(4K) : AT(ADDR(.data) - KERNEL_VIRT_BASE) {
         *(.data .data.*)
     } :data
 
     /* .bss occupies memory but not file space. The loader zeroes it via the
      * gap between each PT_LOAD segment's file size and memory size, so no
-     * boundary symbols are needed here. (Zig's self-hosted linker also does not
-     * yet honour linker-script symbol assignments.) */
-    .bss ALIGN(4K) : {
+     * boundary symbols are needed here. */
+    .bss ALIGN(4K) : AT(ADDR(.bss) - KERNEL_VIRT_BASE) {
         *(.bss .bss.*)
         *(COMMON)
     } :data

src/kernel/main.zig

@@ -40,11 +40,13 @@ var ap_trampoline_page: u64 = 0;
 /// Kernel entry point. The bootloader jumps here after `ExitBootServices` with a
 /// pointer to the handoff data. There is no runtime, no stack unwinding, and no
 /// caller to return to, so this never returns.
-export fn _start(boot_info: *const BootInfo) callconv(kernel_abi) noreturn {
-    // The loader passes a low physical pointer (its own stack). Reach it — and
-    // everything it points at — through the physmap, so it stays valid once the
-    // low identity map is gone. The physmap base is the same under the loader's
-    // bootstrap tables and the kernel's own.
+/// The real entry (`_start`, in isr.s) installs a kernel-owned stack in .bss
+/// then calls this with the loader's `boot_info` pointer in RDI. We can't keep
+/// running on the loader's stack: it's a low physical address that the identity
+/// map covers only transitionally, and vanishes once the kernel drops the low
+/// half. `boot_info` (also low) is reached through the physmap — its base is the
+/// same under the loader's bootstrap tables and the kernel's own.
+export fn kmainEntry(boot_info: *const BootInfo) callconv(kernel_abi) noreturn {
     kmain(@ptrFromInt(danos.physToVirt(@intFromPtr(boot_info))));
 }