move the running task into per-CPU state

PerCpu.current via the GS base; ready queues stay global.

src/kernel/arch/x86_64/cpu.zig

@@ -81,6 +81,24 @@ pub fn readCr3() u64 {
     );
 }
 
+/// IA32_GS_BASE: the hidden base of the GS segment. We repurpose it as the per-CPU
+/// data pointer (there's no user mode yet, so no `swapgs` dance — GS base is always
+/// the running core's per-CPU block). Set once per core during bring-up, after the
+/// GDT is loaded (loading a GS *selector* would otherwise clobber this base).
+const ia32_gs_base = 0xC000_0101;
+
+/// Publish this core's per-CPU data pointer so `cpuLocal` can retrieve it. Each
+/// core calls this once, after its GDT is in place.
+pub fn setCpuLocal(ptr: usize) void {
+    io.wrmsr(ia32_gs_base, ptr);
+}
+
+/// This core's per-CPU data pointer (the value `setCpuLocal` stored). Reads the GS
+/// base MSR — a per-core register, so each core sees its own without any locking.
+pub fn cpuLocal() usize {
+    return io.rdmsr(ia32_gs_base);
+}
+
 /// Kernel tick rate: 1000 Hz (1 ms), the scheduler's time quantum.
 pub const timer_hz = 1000;