schedule tasks across all cores

Per-core GDT/TSS and AP scheduler entry; fix AP SSE + single_threaded.

src/kernel/arch/x86_64/cpu.zig

@@ -114,11 +114,18 @@ pub fn prepareSecondaries(tramp_phys: u64) void {
     smp.prepare(tramp_phys);
 }
 
-/// Wake the core with Local APIC id `apic_id`, giving it `stack_top` and its per-CPU
-/// pointer `percpu`; it adopts the current (kernel) page tables. Returns false if it
-/// doesn't come online within the timeout. Blocks until the core reports in.
-pub fn startSecondary(apic_id: u32, stack_top: usize, percpu: usize) bool {
-    return smp.startAp(apic_id, stack_top, percpu, readCr3());
+/// Wake the core with Local APIC id `apic_id` as dense CPU `index`, giving it
+/// `stack_top` and its per-CPU pointer `percpu`; it adopts the current (kernel) page
+/// tables. Returns false if it doesn't come online within the timeout. Blocks until
+/// the core reports in.
+pub fn startSecondary(apic_id: u32, stack_top: usize, percpu: usize, index: usize) bool {
+    return smp.startAp(apic_id, stack_top, percpu, index, readCr3());
+}
+
+/// Register the generic entry a woken AP jumps to once its arch state is up (its own
+/// descriptor tables, LAPIC, and timer). The kernel passes its scheduler entry here.
+pub fn setSecondaryEntry(entry: *const fn () callconv(.c) noreturn) void {
+    smp.setSecondaryEntry(entry);
 }
 
 /// Kernel tick rate: 1000 Hz (1 ms), the scheduler's time quantum.