[Phase 2.1] GDT user space segments + heap growth

- Restructure GDT to add kernel_data/user_data/user_code in the order
  required for SYSCALL/SYSRET ABI:
    0x08 kernel code, 0x10 kernel data, 0x18 user data, 0x20 user code, 0x28 TSS
  STAR MSR values: STAR[47:32]=0x08, STAR[63:48]=0x10
- Add TSS.privilege_stack_table[0] (RSP0) with 8 KiB static initial stack
  for Ring3→Ring0 hardware interrupt transitions
- Expose GDT static and all Selectors fields as pub (needed by syscall module)
- Add set_kernel_stack(VirtAddr) for scheduler to update RSP0 per-process
- Grow HEAP_SIZE 100 KiB → 4 MiB to support process table + kernel stacks
- Fix pre-existing lifetime elision lint in allocator.rs
- Update flake.nix: add cpio, busybox, gdb, binutils, e2fsprogs
- Update NOTES.md with decisions and next steps

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 19 insertions, 3 deletions

diff --git a/NOTES.md b/NOTES.md
index d140620..5358eb5 100644
--- a/NOTES.md
+++ b/NOTES.md
@@ -17,9 +17,9 @@
 ## Current Status
 
 **Branch**: `CLAUDE_TEST`
-**Phase**: Starting Phase 2 — User Space Foundation
-**Last commit**: `[Step 0] Add PLAN.md and NOTES.md`
-**Next task**: `[Phase 2.1]` — Extend GDT with user space segments
+**Phase**: Phase 2 — User Space Foundation
+**Last commit**: `[Phase 2.1] GDT user space segments + heap growth`
+**Next task**: `[Phase 2.2]` — SYSCALL/SYSRET MSR setup
 
 ---
 
@@ -75,4 +75,20 @@ User address limit:     0x0000_8000_0000_0000 (canonical boundary)
 **Next**: Phase 2.1 — Extend GDT.
 **Decisions**: None new.
 
+### [Phase 2.1] 2026-04-08 — GDT user space segments + heap growth
+**Done**:
+- Restructured `StrixKernel/src/gdt.rs`: added `kernel_data`, `user_data`, `user_code` segments in the correct order for SYSCALL/SYSRET ABI
+- Added `TSS.privilege_stack_table[0]` (RSP0) with an 8 KiB static initial stack
+- Exposed `GDT` static and `Selectors` fields as `pub` for use by syscall setup
+- Added `set_kernel_stack(VirtAddr)` for the scheduler to update RSP0 per-process
+- Grew `HEAP_SIZE` from 100 KiB → 4 MiB in `allocator.rs` (needed for process table)
+- Fixed pre-existing lifetime lint in `allocator.rs`
+- Updated `flake.nix` to add `cpio`, `busybox`, `gdb`, `binutils`, `e2fsprogs`
+- `basic_boot` integration test passes in QEMU via `nix develop`
+**Next**: Phase 2.2 — SYSCALL/SYSRET MSR setup (`src/syscall/mod.rs`)
+**Decisions**:
+- GDT order: kernel_code(0x08) / kernel_data(0x10) / user_data(0x18) / user_code(0x20) / TSS(0x28)
+- STAR MSR: `[47:32]=0x08`, `[63:48]=0x10` → SYSRET CS=0x20, SS=0x18
+- `set_kernel_stack` uses raw pointer write inside `unsafe {}` block; safe when interrupts disabled
+
 ---