Stage MC-0a: per-core block on one core -- current VM moves behind the per-CPU register (FABRIC-3.10.8 §3.1)

- New sk_percpu_t { self, core_id, current_vm }, reached via GS base
  (amd64, %gs:0), TPIDR_EL1 (aarch64), tp (riscv64). Boot core's block is
  one static object installed right after arch_early_init(); secondary
  cores' blocks come with MC-1.
- switch.c's global g_switch_current_vm is gone; current VM is
  sk_percpu()->current_vm, re-read after resume in the context switch.
- Probe (removed): normal path and every heartbeat ISR found core 0's
  block on all three ISAs (ISR ok 889/1877/1625, bad 0).
- Standard sequence on amd64/aarch64/riscv64: hashes unchanged from O-C,
  G A=40 B=40, MINT/WIREBIND/USE ok, conservation true, 0 idle growth.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/starkernel/hal/percpu.c

@@ -0,0 +1,40 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+ See the License for the specific language governing permissions and
+ limitations under the License.
+*/
+
+/**
+ * percpu.c - The boot core's per-core block (FABRIC-3.10.8 Stage MC-0a).
+ * See starkernel/percpu.h.
+ */
+
+#include "starkernel/percpu.h"
+
+/* The boot core's block: one object, not a table sized to a core count.
+ * Secondary cores get theirs as they come online (MC-1). */
+static sk_percpu_t g_percpu_boot;
+
+void sk_percpu_boot_init(void)
+{
+    g_percpu_boot.self       = &g_percpu_boot;
+    g_percpu_boot.core_id    = 0;
+    g_percpu_boot.current_vm = (struct VM *)0;
+    sk_percpu_install(&g_percpu_boot);
+}

src/starkernel/kernel_main.c

@@ -66,6 +66,7 @@ EFI_RUNTIME_SERVICES *g_sk_runtime_services = NULL;
 #include "starkernel/capsule_zuse_boot.h" /* capsule_zuse_boot_load_root_pubkey */
 #include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 3 */
 #include "starkernel/vm/switch.h" /* sk_vm_switch_set_current() -- Stage 3 follow-on */
+#include "starkernel/percpu.h"    /* FABRIC-3.10.8 MC-0a -- sk_percpu_boot_init() */
 #include "starkernel/artemis_sig.h" /* artemis_sig_check/genesis_stamp */
 #include "starkernel/kmalloc.h"
 #include "starkernel/repl.h"
@@ -393,6 +394,12 @@ void kernel_main(BootInfo *boot_info) {
      */
     arch_early_init();
 
+    /* FABRIC-3.10.8 Stage MC-0a: this core's per-core block, installed in
+     * the per-CPU register. Straight after arch_early_init() -- on amd64 its
+     * GS reload resets the GS base -- and before anything that can switch or
+     * take an interrupt. */
+    sk_percpu_boot_init();
+
     /* M1: Console initialization — serial UART first */
     console_init();
     print_banner();

src/starkernel/vm/switch.c

@@ -34,6 +34,7 @@
 #include "starkernel/hal/hal.h" /* sk_hal_panic() */
 #include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.8.md §3.13.3 -- the decision */
 #include "starkernel/vm/bootstrap/sk_vm_bootstrap.h" /* sk_get_mama_vm() -- the idle root */
+#include "starkernel/percpu.h" /* FABRIC-3.10.8 MC-0a -- this core's current VM */
 #include <stdint.h>
 
 /* Arch-specific (switch.S in each arch dir). sk_vm_switch_prime() builds
@@ -70,11 +71,14 @@ static VM *g_switch_entry_vm;
  * Seeded once at boot via sk_vm_switch_set_current() for whichever VM is
  * running before any switch has ever happened (Hera, at Stage 3
  * registration time -- kernel_main.c). */
-static VM *g_switch_current_vm;
+/* FABRIC-3.10.8 Stage MC-0a: "who is running" is per core now, held in this
+ * core's per-core block (starkernel/percpu.h) instead of the file-scope
+ * global g_switch_current_vm this comment used to describe. Same semantics
+ * on one core; with more, each core answers for itself. */
 
-VM *sk_vm_switch_current_vm(void) { return g_switch_current_vm; }
+VM *sk_vm_switch_current_vm(void) { return sk_percpu()->current_vm; }
 
-void sk_vm_switch_set_current(VM *vm) { g_switch_current_vm = vm; }
+void sk_vm_switch_set_current(VM *vm) { sk_percpu()->current_vm = vm; }
 
 /**
  * sk_vm_switch_entry - trampoline for a VM's first-ever entry.
@@ -219,7 +223,7 @@ void sk_vm_switch_checkpoint(VM *vm) {
  * mail or pending console input, or is returned to by a trampoline with
  * nothing left to do. */
 void sk_vm_idle_point(int may_drain_self) {
-    VM *self = g_switch_current_vm;
+    VM *self = sk_percpu()->current_vm;
     VM *next;
 
     if (!self || self != (VM *)sk_get_mama_vm()) return;
@@ -274,14 +278,16 @@ int sk_vm_context_switch(VM *from, VM *to) {
     capsule_vm_set_state(to->stadium_vm_id, VM_STATE_LIVE);
     capsule_vm_set_state(from->stadium_vm_id, VM_STATE_SWITCHED_OUT);
     sk_vm_switch_signal_note_parked(from->stadium_vm_id); /* §3.13.3 wait stamp */
-    g_switch_current_vm = to;
+    sk_percpu()->current_vm = to;
 
     sk_vm_switch_to(&from->native_stack_saved_sp, new_sp);
 
     /* Resumes here only once something later switches back to `from` --
-     * from `from`'s own point of view, this call simply took a while. */
+     * from `from`'s own point of view, this call simply took a while.
+     * sk_percpu() is read fresh here, not cached from before the switch:
+     * once workers migrate (MC-3), `from` may resume on a different core. */
     capsule_vm_set_state(from->stadium_vm_id, VM_STATE_LIVE);
-    g_switch_current_vm = from;
+    sk_percpu()->current_vm = from;
     return 0;
 }