refactor: tasks code cleanup

src/arch/x86_64/mod.rs

@@ -202,7 +202,6 @@ pub unsafe fn enter_user(
             "mov ds, {user_data_selector:x}",
             "mov es, {user_data_selector:x}",
             "mov fs, {user_data_selector:x}",
-            "mov gs, {user_data_selector:x}",
 
             "push {user_data_selector}",
             "push {user_stack_pointer}",
@@ -227,6 +226,10 @@ pub unsafe fn enter_user(
             "xor r14, r14",
             "xor r15, r15",
 
+            // Kernel GS is CpuLocal; leave it in IA32_KERNEL_GS_BASE so
+            // syscall_entry can recover it with SWAPGS.
+            "swapgs",
+            "mov gs, {user_data_selector:x}",
             "iretq",
             user_data_selector = in(reg) gdt::USER_DATA_SELECTOR as usize,
             user_code_selector = in(reg) gdt::USER_CODE_SELECTOR as usize,

src/arch/x86_64/syscall.rs

@@ -44,6 +44,10 @@ pub fn init(cpu_local: *const CpuLocal) {
         write_msr(IA32_FMASK, RFLAGS_MASK);
         write_msr(IA32_GS_BASE, 0);
         write_msr(IA32_KERNEL_GS_BASE, cpu_local as u64);
+
+        // Kernel code always runs with GS pointing at CpuLocal. User entry
+        // swaps this into IA32_KERNEL_GS_BASE before transitioning to ring 3.
+        asm!("swapgs", options(nostack, preserves_flags));
     }
 }
 
@@ -103,7 +107,7 @@ unsafe extern "C" fn syscall_entry() {
 }
 
 extern "C" fn syscall_dispatch(frame: &mut SyscallFrame) {
-    let ret = crate::task::syscall::handle(
+    let ret = crate::syscall::handle(
         frame.rax, frame.rdi, frame.rsi, frame.rdx, frame.r10, frame.r8, frame.r9,
     );