[Phase 2.2/2.3] SYSCALL/SYSRET MSR setup + syscall dispatcher

syscall/mod.rs:
- Configure STAR (kernel CS=0x08, user base=0x10), LSTAR, SFMASK (clear IF),
  EFER.SCE to enable the SYSCALL/SYSRET instruction pair
- Naked assembly entry stub: swapgs, save/restore user RSP via per-CPU static,
  remap r10→rcx for Linux arg4 convention, call Rust syscall_handler, sysretq
- validate_user_ptr(): bounds check for all user pointers (0..USER_ADDR_MAX)
- set_syscall_kernel_stack(): for scheduler to update per-CPU kernel RSP

syscall/dispatch.rs:
- dispatch(): routes syscall numbers; unknown → -ENOSYS (never panics)
- sys_write(fd, buf, count): fd 1/2 → serial; validates user ptr; read_volatile
  loop avoids Rust aliasing assumptions on untrusted memory
- sys_exit(): serial log + hlt_loop() placeholder; Phase 2.5 adds scheduling

lib.rs: register syscall::init() + syscall::init_kernel_stack() in strix_os::init()

All existing integration tests pass.

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

1 files changed, 16 insertions, 2 deletions

diff --git a/NOTES.md b/NOTES.md
index 5358eb5..3463729 100644
--- a/NOTES.md
+++ b/NOTES.md
@@ -18,8 +18,8 @@
 
 **Branch**: `CLAUDE_TEST`
 **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
+**Last commit**: `[Phase 2.2/2.3] SYSCALL/SYSRET MSR setup + syscall dispatcher`
+**Next task**: `[Phase 2.4]` — Process structure (task/process.rs)
 
 ---
 
@@ -75,6 +75,20 @@ User address limit:     0x0000_8000_0000_0000 (canonical boundary)
 **Next**: Phase 2.1 — Extend GDT.
 **Decisions**: None new.
 
+### [Phase 2.2/2.3] 2026-04-08 — SYSCALL/SYSRET MSR setup + dispatcher
+**Done**:
+- `src/syscall/mod.rs`: configure STAR/LSTAR/SFMASK/EFER.SCE MSRs
+- Naked assembly `syscall_entry_asm`: swapgs, save user RSP, load kernel RSP, remap r10→rcx for arg4, call `syscall_handler`, restore, sysretq
+- `src/syscall/dispatch.rs`: dispatcher with `write(1)`, `exit(60)`, `exit_group(231)`; unknown → `-ENOSYS`
+- `sys_write`: validates user pointer before dereference; uses `read_volatile` to avoid aliasing UB
+- Registered `syscall::init()` + `syscall::init_kernel_stack()` in `strix_os::init()`
+- All existing integration tests still pass in QEMU
+**Next**: Phase 2.4 — Process structure
+**Decisions**:
+- Use `swapgs` pattern for user/kernel GS switching (per-CPU scratch for user RSP)
+- `sys_write` uses raw pointer + `read_volatile` loop (not `&[u8]` slice) on user memory
+- `sys_exit` currently halts; Phase 2.5 will add proper process termination
+
 ### [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