LithosAnanake/include/starkernel/capsule_vm_switch_signal.h

/*
  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.
*/

/**
 * capsule_vm_switch_signal.h - New, purpose-built "who runs next" signal
 * for preemptive context switching (FABRIC-3.md §XXVIII, Stage 3,
 * 2026-09-13).
 *
 * Deliberately NOT a repurposing of capsule_vm_physics.c's execution-heat
 * engine -- that measures word-level dispatch fairness over millions of
 * executions on a different timescale, and its own header explicitly
 * documents it as never touched from interrupt context (unlocked, by
 * design). This is a different physical quantity: instant-by-instant
 * run-readiness, consulted from real ISR context (heartbeat_tick()) every
 * timer tick.
 *
 * Concurrency discipline mirrors heartbeat.c's own §21.1-sanctioned
 * pattern for heartbeat_next_period_ns(): single-writer-ISR (tick()) /
 * single-reader-mainline (take_pending(), called from the cooperative
 * checkpoint in execute_colon_word()), no lock, because nothing on this
 * single hart is concurrent with the ISR while it runs.
 *
 * NOT truly interrupt-driven register/stack swapping (that was
 * considered and rejected for this stage -- see FABRIC-3.md §XXVIII
 * Stage 3 for why): the ISR only ever sets a flag. The actual switch
 * (Stage 2's already-proven sk_vm_context_switch()) happens later, at a
 * safe cooperative checkpoint on the mainline, once per word dispatch.
 *
 * Slot table is sized with headroom, not hardcoded to exactly today's 3
 * participants (Hera/Hermes/Artemis) -- extending participation later
 * (Stage 4+) is another sk_vm_switch_signal_register() call, not a
 * redesign.
 *
 * FABRIC-3.6.md task 3.1 (2026-09-21, ruled B2 / FABRIC-3.5.md §XLV.2): the
 * slot table is no longer a fixed SK_SWITCH_MAX_SLOTS=16 compile-time array.
 * It is kmalloc'd at boot by sk_vm_switch_signal_boot_init(), sized from
 * stadium_max_vm_count() -- the same RAM-derived population bound Stadium
 * and session.c already use (session_boot_init() is the direct precedent
 * mirrored here). Every switch-signal participant is a Stadium VM, so
 * reusing that bound directly (rather than re-deriving a separate RAM
 * budget) needs no new sizing formula.
 */

#ifndef STARKERNEL_CAPSULE_VM_SWITCH_SIGNAL_H
#define STARKERNEL_CAPSULE_VM_SWITCH_SIGNAL_H

#ifdef __STARKERNEL__

#include <stdint.h>
#include "starkernel/vm_uuid.h"

/* Boot-time allocation (FABRIC-3.6.md task 3.1, 2026-09-21): kmalloc's the
 * slot table to stadium_max_vm_count() entries. Must run after
 * stadium_boot_init() (that bound is 0, and this fails, until Stadium has
 * computed it) and before the first sk_vm_switch_signal_register() call.
 * Soft failure -- returns -1 and leaves the table unallocated (capacity 0,
 * so register() below simply refuses every registration) rather than
 * halting boot, same posture as stadium_boot_init()/session_boot_init().
 * Idempotent-unsafe: calling twice leaks the first allocation, so callers
 * must call it exactly once. */
int sk_vm_switch_signal_boot_init(void);

/* Register a VM as a switch-signal participant. Returns its slot index,
 * or -1 if the slot table is full (or sk_vm_switch_signal_boot_init() was
 * never called / failed). Call once per participating VM, after that VM is
 * fully born (never mid-birth -- this stage has no critical-section
 * protection against being switched away mid-setup). */
int sk_vm_switch_signal_register(VMUuid vm_id);

/* Remove a switch-signal participant (FABRIC-3.md §XXVIII Stage 4,
 * 2026-09-14) -- Tripod VMs never need this (they live forever), but
 * WIREBIND-birthed identity VMs cycle through attach/detach repeatedly
 * and must free their slot for reuse, or the bounded table exhausts
 * after sk_vm_switch_signal_slot_capacity() attach/detach cycles. Compacts the table
 * (small, bounded, mutated only at attach/detach -- not a hot path).
 * Clears a pending switch targeting this VM, if any, so the checkpoint
 * never attempts to switch into a no-longer-registered participant.
 * No-op (returns -1) if vm_id was never registered. */
int sk_vm_switch_signal_unregister(VMUuid vm_id);

/* Called from heartbeat_tick() (ISR context) every timer tick. Cheap:
 * iterates only the registered slots (bounded, small). */
void sk_vm_switch_signal_tick(void);

/* Called from the cooperative checkpoint (execute_colon_word(), mainline,
 * once per word dispatch). Returns the VMUuid of a VM that should now be
 * switched to, or vm_uuid_none() if nothing is pending. Clears the
 * pending flag as a side effect -- call at most once per checkpoint. */
VMUuid sk_vm_switch_signal_take_pending(void);

/* Call once, from the same checkpoint, immediately after a switch
 * sk_vm_switch_signal_take_pending() requested actually executes (not if
 * the target turned out invalid/self) -- feeds the DoE CSV counters
 * below. Also resets `target_id`'s own readiness/has_work directly
 * (Stage 3 follow-on correction, 2026-09-14) -- see the .c file's own doc
 * comment on why this can't be left to tick()'s current-slot polling. */
void sk_vm_switch_signal_note_switch_performed(VMUuid target_id);

/* Message-arrival eligibility hook (FABRIC-3.md §XXVIII Stage 3 follow-on,
 * 2026-09-14): mark that VM `vm_id` was just sent a message (the FORTH-side
 * MSG-SEND hook in capsules/common/messaging.4th calls this via the new
 * SWITCH-MARK-WORK primitive, mainline, single-writer). Consulted by
 * sk_vm_switch_signal_tick()'s own readiness->pending decision so a VM
 * with nothing recently sent to it never becomes a switch target purely by
 * sitting idle long enough -- closes the wasteful (but, since the Stage 3
 * stack-ownership fix, no longer corrupting) trampoline-bounce cycle for
 * an idle participant. No-op for an unregistered vm_id. */
void sk_vm_switch_signal_mark_work(VMUuid vm_id);

/* DoE CSV read-only exposure (FABRIC-3.md §XXVIII Stage 3 follow-on,
 * 2026-09-13) -- all of this state already existed for the switch
 * decision itself; these just make it observable. */
uint64_t sk_vm_switch_signal_switch_count(void);      /* cumulative, since boot */
uint32_t sk_vm_switch_signal_ticks_since_switch(void);
int      sk_vm_switch_signal_current_slot(void);       /* -1 = none/unregistered */
int      sk_vm_switch_signal_slot_count(void);
uint32_t sk_vm_switch_signal_readiness(int slot);      /* 0 if slot out of range */
uint32_t sk_vm_switch_signal_readiness_of(VMUuid vm_id); /* 0 if not registered */

/* FABRIC-3.6.md task 3.1 (2026-09-21): the table's boot-time-computed
 * capacity (0 if sk_vm_switch_signal_boot_init() was never called or
 * failed) -- the dynamic replacement for the old compile-time
 * SK_SWITCH_MAX_SLOTS=16. */
int      sk_vm_switch_signal_slot_capacity(void);

#endif /* __STARKERNEL__ */

#endif /* STARKERNEL_CAPSULE_VM_SWITCH_SIGNAL_H */