LithosAnanake/kernel/src/heartbeat.c

/*
  StarForth — Steady-State Virtual Machine Runtime

  Copyright (c) 2023–2025 Robert A. James
  All rights reserved.

  Licensed under the StarForth License, Version 1.0
*/

/**
 * heartbeat.c - Shared M5 heartbeat / TIME-TRUST engine (punch-list item 0.8)
 *
 * One implementation of heartbeat_init()/heartbeat_ticks()/heartbeat_trust()/
 * heartbeat_state(), shared by all three architectures. Each architecture's
 * timer.c contributes only heartbeat_read_counter() — the one thing that is
 * genuinely per-ISA (rdtsc / rdtime / CNTPCT_EL0).
 *
 * Top half / bottom half split (FABRIC-0.md §25.1 item 0.8, per the GAP-A1
 * ruling in §16.4/§18.4): heartbeat_tick() is called from interrupt context
 * and does nothing but read the counter, bump TIME-TICKS, and latch a
 * pending sample. heartbeat_service() runs on the mainline (the REPL idle
 * loop) and does the window/variance/trust work. Neither one feeds patron
 * state — the engine stays on the virtual tick per §18.4, unchanged by this
 * file.
 *
 * Adaptive re-arm period (FABRIC-0.md §26, ruled 2026-08-03): Loop #7
 * (vm_runtime.c) computes an execution-derived stable/volatile signal and
 * calls heartbeat_set_adaptive_period_ns() with it, rescaled to this file's
 * kernel-appropriate base (10 ms, matching the 100 Hz rate item 0.1-0.7
 * configured) rather than the hosted 10 µs HEARTBEAT_TICK_NS base. Each
 * architecture's re-arm function reads heartbeat_next_period_ns() and
 * converts it to raw counter units instead of using a fixed constant.
 * Single writer (mainline, via vm_tick()'s Loop #7 site), single reader
 * (the ISR's re-arm call) — no lock needed, per §21.1's finding that
 * nothing here is actually concurrent on one hart.
 */

#include "starkernel/timer.h"
#include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 3 */

#if defined(ARCH_AMD64)
#define HEARTBEAT_HAS_VARIANCE 1
#else
#define HEARTBEAT_HAS_VARIANCE 0
#endif

/* Kernel-side adaptive-period base: 10 ms, matching the 100 Hz hardware
 * rate established throughout items 0.1-0.7. Deliberately NOT
 * HEARTBEAT_TICK_NS (include/starforth_config.h) -- that constant is the
 * hosted pthread-worker's 10 µs base and is three orders of magnitude too
 * fast for a bare-metal ISR period (FABRIC-0.md §26.3). Loop #7's decision
 * logic is reused unmodified; only the base it scales differs. */
#define HEARTBEAT_BASE_PERIOD_NS  10000000ULL

static TimeTrustState g_heartbeat;

/* Top/bottom half handoff. */
static volatile uint64_t g_pending_counter;
static volatile int      g_pending_valid;

/* Adaptive re-arm period, clamped to the same [1/4x, 4x] band Loop #7
 * itself enforces on tick_target_ns, so a caller cannot runaway the ISR
 * rate even if it forwarded an unclamped value. */
static volatile uint64_t g_adaptive_period_ns = HEARTBEAT_BASE_PERIOD_NS;

#if HEARTBEAT_HAS_VARIANCE

/**
 * @brief Push a signed delta value into the heartbeat rolling window.
 */
static void window_push(TimeWindow *w, int64_t delta)
{
    w->deltas[w->pos] = delta;
    w->pos = (w->pos + 1) % TIME_WINDOW_SIZE;
    if (w->count < TIME_WINDOW_SIZE) {
        w->count++;
    }
}

/**
 * @brief Compute the relative variance of heartbeat deltas as a Q48.16 value.
 */
static q48_16_t window_variance_q48(const TimeWindow *w, uint64_t expected_delta)
{
    if (w->count < 2 || expected_delta == 0) {
        return 0;
    }

    int64_t sum = 0;
    for (uint32_t i = 0; i < w->count; i++) {
        sum += w->deltas[i];
    }
    int64_t mean = sum / (int64_t)w->count;

    uint64_t sum_sq = 0;
    for (uint32_t i = 0; i < w->count; i++) {
        int64_t diff = w->deltas[i] - mean;
        if (diff > 0x7FFFFFFF) diff = 0x7FFFFFFF;
        if (diff < -0x7FFFFFFF) diff = -0x7FFFFFFF;
        sum_sq += (uint64_t)(diff * diff);
    }
    uint64_t var_tsc = sum_sq / w->count;

    uint64_t exp_sq = expected_delta;
    if (exp_sq > 0xFFFFFFFF) {
        var_tsc >>= 16;
        exp_sq >>= 8;
    }
    exp_sq = exp_sq * exp_sq;
    if (exp_sq == 0) return 0;

    if (var_tsc > 0x0000FFFFFFFFFFFFULL) {
        var_tsc = 0x0000FFFFFFFFFFFFULL;
    }

    return (var_tsc << 16) / exp_sq;
}

/**
 * @brief Derive TIME-TRUST from a Q48.16 relative variance value.
 */
static q48_16_t variance_to_trust(q48_16_t variance)
{
    q48_16_t denom = q48_add(Q48_ONE, variance);
    if (denom == 0) {
        return Q48_ONE;
    }
    return q48_div(Q48_ONE, denom);
}

#endif /* HEARTBEAT_HAS_VARIANCE */

/**
 * @brief Initialise the M5 heartbeat / TIME-TRUST subsystem.
 *
 * Shared across all three architectures -- the fallback and calibration
 * formula never differed between them, only what heartbeat_service() does
 * with the resulting window did.
 */
void heartbeat_init(uint64_t tsc_hz, uint64_t tick_hz)
{
    g_heartbeat.ticks = 0;
    g_heartbeat.last_tsc = 0;
    g_heartbeat.total_samples = 0;
    g_heartbeat.variance = 0;
    g_heartbeat.trust = Q48_ONE;

    g_heartbeat.window.pos = 0;
    g_heartbeat.window.count = 0;
    for (int i = 0; i < TIME_WINDOW_SIZE; i++) {
        g_heartbeat.window.deltas[i] = 0;
    }

    g_heartbeat.expected_delta = (tick_hz > 0 && tsc_hz > 0)
        ? (tsc_hz / tick_hz) : 10000000ULL;

    g_pending_valid = 0;
    g_adaptive_period_ns = HEARTBEAT_BASE_PERIOD_NS;
}

void heartbeat_tick(void)
{
    g_pending_counter = heartbeat_read_counter();
    g_heartbeat.ticks++;
    g_pending_valid = 1;

    /* FABRIC-3.md §XXVIII, Stage 3 (2026-09-13): the one deliberate,
     * documented amendment to this file's own top-half/bottom-half
     * discipline above. sk_vm_switch_signal_tick() reaches into VM
     * scheduling state from real ISR context -- something this file's
     * header comment previously said nothing here did. It stays cheap
     * (bounded iteration over a small slot table, no locking needed for
     * the same reason nothing else here needs it: single hart, nothing
     * concurrent with this ISR while it runs) and it never itself
     * switches anything -- it only ever sets a flag, consumed later at a
     * safe cooperative checkpoint on the mainline
     * (execute_colon_word()). The GAP-A1 split itself is not overturned:
     * everything else in this function is exactly as minimal as before. */
    sk_vm_switch_signal_tick();
}

void heartbeat_service(void)
{
    if (!g_pending_valid) {
        return;
    }
    uint64_t now = g_pending_counter;
    g_pending_valid = 0;

    TimeTrustState *s = &g_heartbeat;
    s->total_samples++;

    if (s->total_samples == 1) {
        s->last_tsc = now;
        return;
    }

    uint64_t actual_delta = now - s->last_tsc;
    s->last_tsc = now;
    int64_t deviation = (int64_t)actual_delta - (int64_t)s->expected_delta;

#if HEARTBEAT_HAS_VARIANCE
    window_push(&s->window, deviation);
    s->variance = window_variance_q48(&s->window, s->expected_delta);
    s->trust = variance_to_trust(s->variance);
#else
    /* RISC-V `time` / AArch64 CNTPCT_EL0 are architecturally invariant
     * counters -- no statistical quality estimate needed (matches the
     * per-arch rationale this file replaces). */
    (void)deviation;
    s->trust = Q48_ONE;
#endif
}

uint64_t heartbeat_ticks(void)
{
    return g_heartbeat.ticks;
}

time_trust_t heartbeat_trust(void)
{
    return g_heartbeat.trust;
}

const TimeTrustState *heartbeat_state(void)
{
    return &g_heartbeat;
}

/**
 * @brief Set the adaptive re-arm period (FABRIC-0.md §26).
 *
 * Called from vm_runtime.c's Loop #7 site on the mainline execution path
 * (never interrupt context) with a value already rescaled to this file's
 * HEARTBEAT_BASE_PERIOD_NS. Clamped defensively to the same [1/4x, 4x]
 * band Loop #7 itself enforces around its own base, so a caller forwarding
 * an unclamped or wrongly-scaled value cannot run the ISR away.
 *
 * @param ns Desired period in nanoseconds for the next re-arm.
 */
void heartbeat_set_adaptive_period_ns(uint64_t ns)
{
    uint64_t lo = HEARTBEAT_BASE_PERIOD_NS / 4;
    uint64_t hi = HEARTBEAT_BASE_PERIOD_NS * 4;
    if (ns < lo) ns = lo;
    if (ns > hi) ns = hi;
    g_adaptive_period_ns = ns;
}

/**
 * @brief Read the period the next hardware re-arm should use.
 *
 * Called from interrupt context by each architecture's re-arm function
 * (apic_timer_rearm(), riscv64_timer_rearm(), the AArch64 equivalent) in
 * place of the fixed constant those functions used before item 0.8.
 *
 * @return Current adaptive period in nanoseconds.
 */
uint64_t heartbeat_next_period_ns(void)
{
    return g_adaptive_period_ns;
}