feat(memctx): persist kcr3 to fast-restart without a cold rescan

The cold host_bootstrap hunts the agent beacon across physical RAM and is
slow and unstable: after a restart the adapter re-scans from scratch, minutes
in which there is no address-space context to vend, though the guest is long
booted and its System DTB (kcr3) is unchanged.

Cache the kcr3 from a successful live scan in a watch-dir sibling of the slot
map (tmpfs: survives a restart, dies with the RAM file on host reboot). On
attach, re-validate the cached kcr3 against the live RAM via an O_RDONLY
context (open_ro_fd, which bypasses the beacon scan) plus a System-cr3 match,
and publish the read datum immediately when it still resolves the kernel. A
guest reboot changes the System DTB, so a stale kcr3 no longer resolves and
falls back to a cold scan: the boot-session discriminator is the kcr3 itself,
not file metadata.

The gva_write target is never taken from the cache: it is set only by a fresh
live scan, so a persisted kcr3 is a read locator only and MEMWRITE stays
fail-closed until a cold bootstrap acquires the write hold.

Persist is off unless the path is supplied (NULL keeps current behaviour).

Bump 0.3.12.

CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.16)
 # Single source of truth for the version: CI passes -DVMSIG_VERSION=${TAG#v}, so the project
 # version (-> libvgpu-perception SONAME/.so version) and the .deb version come from one tag.
-set(VMSIG_VERSION "0.3.11" CACHE STRING "Release version (MAJOR.MINOR.PATCH); CI passes the tag")
+set(VMSIG_VERSION "0.3.12" CACHE STRING "Release version (MAJOR.MINOR.PATCH); CI passes the tag")
 project(vmsig VERSION ${VMSIG_VERSION} LANGUAGES C)
 
 set(CMAKE_C_STANDARD 17)

src/adapter/memctx/include/memctx.h

@@ -15,6 +15,13 @@ typedef struct {
     uint32_t    fail_boots; /* test-only: fail the first N stub bootstraps before        */
                             /* succeeding (drives the retry/backoff path deterministically */
                             /* without timing dependence); 0 in production. stub path only. */
+    const char* persist_path; /* armed: path to the kcr3 cache file (sibling of .slots in the */
+                            /* watch dir, tmpfs-local: survives a daemon restart, dies with the */
+                            /* RAM file on host reboot). NULL/empty => persist disabled (cold   */
+                            /* bootstrap only). The boot-session discriminator is the kcr3      */
+                            /* itself: on resume it is validated against live RAM via            */
+                            /* vmie_win32_open_ro_fd (NULL if it no longer resolves the kernel) */
+                            /* — a stale kcr3 after a guest reboot is rejected, fail-closed.    */
 } vmsig_memctx_cfg;
 
 /* Max SRC bytes per atomic gva_write (bounds the worker POD slot; mc_req header + src

src/adapter/memctx/memctx.c

@@ -26,6 +26,7 @@
 #include <sys/mman.h>
 #include <sys/epoll.h>
 #include <sys/timerfd.h>       /* one-shot backoff timer for cold-bootstrap retry */
+#include <sys/stat.h>          /* persist file mode bits (0600) */
 
 #ifdef VMSIG_WITH_VMIE
 #include "win32.h"              /* vmie_win32_open/host_bootstrap/proc_list/close */
@@ -67,7 +68,82 @@ static int memfd_create(const char* name, unsigned int flags) {
  * eventfd, slot 1 is the one-shot backoff timerfd that re-kicks the bootstrap. */
 enum { MC_COOKIE_WORKER = 0, MC_COOKIE_RETRY = 1 };
 
-enum { MC_JOB_BOOTSTRAP = 0, MC_JOB_WRITE = 1 };
+/* MC_JOB_RESUME: fast-path boot-session re-validation. On a daemon restart the cold scan
+ * (host_bootstrap) is slow AND unstable (it hunts the agent beacon across physical RAM); if
+ * the guest did NOT reboot, its System DTB (kcr3) is unchanged and was cached at the last
+ * live scan. RESUME re-opens an O_RDONLY context with that cached kcr3 (vmie_win32_open_ro_fd,
+ * which bypasses the beacon scan) — the boot-session discriminator is the kcr3 ITSELF against
+ * the live RAM: it resolves the kernel (ntoskrnl) only if the guest is the same boot. */
+enum { MC_JOB_BOOTSTRAP = 0, MC_JOB_WRITE = 1, MC_JOB_RESUME = 2 };
+
+/* ---- kcr3 context persist: a cache of the cold-bootstrap result, mirror of the .slots
+ * idiom in src/discovery/slot.c (magic+version POD, native byte order, atomic tmp+rename,
+ * fail-soft load). Deliberately NOT factored into a shared helper: discovery (vmid<->slot)
+ * and this adapter (kcr3 cache) are different layers with different lifecycles — Rule-of-three
+ * is not reached, and a shared helper would couple the two prematurely.
+ *
+ * We persist the MINIMUM: only {magic, version, kcr3}. NO RAM metadata (st_ino/size/mtime/
+ * btime): those do NOT prove the RAM holds the same boot session (the backing file outlives a
+ * memory overwrite, the inode can be reused). The boot-session discriminator is the kcr3
+ * self-validating against the live RAM at load time (see MC_JOB_RESUME), not file metadata.
+ *
+ * MEMWRITE-target safety: a persisted kcr3 is a READ locator only. The write target (a->kcr3)
+ * is set ONLY by the bootstrap worker after a fresh live scan — never from this file. */
+#define MC_PERSIST_MAGIC   0x4B435258u   /* "KCRX" — kcr3 context cache */
+#define MC_PERSIST_VERSION 1u
+typedef struct {
+    uint32_t magic;
+    uint32_t version;
+    uint64_t kcr3;        /* System DTB obtained from a live RAM scan; validated by open_ro_fd */
+} mc_persist_blob;
+
+/* Atomic save: write a temp sibling then rename over the target, so a reader (or a racing
+ * second daemon) sees either the whole old file or the whole new one. Loop-thread-only.
+ * Returns 0 on success, -1 otherwise (best-effort: the datum is already published). */
+static int mc_persist_save(const char* path, uint64_t kcr3) {
+    if (!path || !*path) return -1;
+    mc_persist_blob b;
+    memset(&b, 0, sizeof b);
+    b.magic = MC_PERSIST_MAGIC; b.version = MC_PERSIST_VERSION; b.kcr3 = kcr3;
+
+    char tmp[512];
+    int n = snprintf(tmp, sizeof tmp, "%s.tmp", path);
+    if (n < 0 || (size_t)n >= sizeof tmp) return -1;
+
+    int fd = open(tmp, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, 0600);
+    if (fd < 0) return -1;
+    ssize_t w = write(fd, &b, sizeof b);
+    int rc = (w == (ssize_t)sizeof b) ? 0 : -1;
+    if (close(fd) != 0) rc = -1;
+    if (rc == 0 && rename(tmp, path) != 0) rc = -1;
+    if (rc != 0) unlink(tmp);
+    return rc;
+}
+
+/* Load + validate the POD header. Loop-thread-only. Returns 1 if a well-formed blob was read
+ * (out filled), 0 otherwise (no file / short / wrong magic or version => fail-soft, fall back
+ * to a cold bootstrap). No migrations: an old version is ignored and overwritten by the next
+ * live scan result. NOTE: this validates only the file SHAPE; the kcr3 itself is validated
+ * against live RAM on the worker (MC_JOB_RESUME), which is the real boot-session discriminator. */
+static int mc_persist_load(const char* path, mc_persist_blob* out) {
+    if (!path || !*path) return 0;
+    int fd = open(path, O_RDONLY | O_CLOEXEC);
+    if (fd < 0) return 0;                          /* no file => cold bootstrap */
+    mc_persist_blob b;
+    ssize_t r = read(fd, &b, sizeof b);
+    close(fd);
+    if (r != (ssize_t)sizeof b || b.magic != MC_PERSIST_MAGIC || b.version != MC_PERSIST_VERSION)
+        return 0;                                  /* corrupt/old => cold bootstrap */
+    *out = b;
+    return 1;
+}
+
+/* Drop the cache on a destructive VM-lifecycle (the RAM may have changed). Best-effort.
+ * Hygiene only: even without the drop a stale kcr3 would be rejected by the self-validation,
+ * but we do not leave a known-dead file around. Loop-thread-only. */
+static void mc_persist_drop(const char* path) {
+    if (path && *path) unlink(path);
+}
 
 /* worker req/res (POD <= VMSIG_WORK_SLOT). One off-loop worker runs BOTH the cold
  * bootstrap and the atomic writes (FIFO serializes a write against the close-on-rebootstrap).
@@ -80,8 +156,9 @@ typedef struct {
     uint32_t attempt;     /* MC_JOB_BOOTSTRAP: consecutive-failure index of THIS  */
                           /* kick (copy of a->boot_attempts); stub fails while    */
                           /* attempt < a->fail_boots. NOT the epoch counter.      */
-    /* --- MC_JOB_WRITE --- */
+    /* --- MC_JOB_WRITE / MC_JOB_RESUME --- */
-    uint64_t cr3;         /* target AS root; 0 => a->kcr3 (kernel AS), resolved on worker */
+    uint64_t cr3;         /* WRITE: target AS root (0 => a->kcr3); RESUME: persisted kcr3 to validate */
+    uint64_t low;         /* MC_JOB_RESUME: below-4G split for vmie_win32_open_ro_fd (ignored by others) */
     uint64_t gva;
     uint32_t len;
     uint32_t corr;
@@ -100,6 +177,7 @@ struct vmsig_adapter {
     uint32_t      endpoint;
     int           stub;
     const char*   ram_path;       /* armed: RAM-backing path (NOT published outward) */
+    const char*   persist_path;   /* armed: kcr3 cache file path (cfg, loop-thread-only); NULL => persist off */
     uint64_t      low;
     int           cfg_ro_fd;      /* >=0 => infra-sealed RO-fd (owned by adapter, closed in mc_close); <0 => default */
     vmsig_emit    emit;
@@ -232,6 +310,49 @@ static int mc_job(void* user, const void* req, void* res) {
 #endif
     }
 
+    if (rq->op == MC_JOB_RESUME) {
+        /* Fast-path boot-session re-validation: open an O_RDONLY context with the PERSISTED
+         * kcr3 and let the engine decide if it still resolves the kernel in the LIVE RAM.
+         * This is purely a READ validation — it NEVER touches a->win/a->mem/a->kcr3 (the
+         * RW write-hold, owned by the bootstrap worker after a fresh live scan). MEMWRITE-
+         * target safety: a persisted kcr3 must never become the gva_write target. */
+        if (a->stub) {
+            /* No VMIE here, so there is no real RAM to validate against: synthetically ACCEPT a
+             * nonzero kcr3 so the stub can exercise the persist MECHANICS (save/load/fast-vs-slow
+             * selection). This is NOT real boot-session validation — that is armed-only. */
+            if (rq->cr3 == 0) return -1;
+            rs->kcr3 = rq->cr3;
+            return 0;
+        }
+#ifdef VMSIG_WITH_VMIE
+        /* fresh O_RDONLY fd over the backing (same source as mc_reg_share_fd: dup the infra
+         * RO-fd, else open ram_path O_RDONLY). The RO context borrows it (dup'd internally),
+         * so we close our copy after open. */
+        int rfd;
+        if (a->cfg_ro_fd >= 0) rfd = fcntl(a->cfg_ro_fd, F_DUPFD_CLOEXEC, 0);
+        else if (a->ram_path) rfd = open(a->ram_path, O_RDONLY | O_CLOEXEC);
+        else return -1;
+        if (rfd < 0) return -1;
+        vmie_win32* v = vmie_win32_open_ro_fd(rfd, rq->low, rq->cr3);
+        close(rfd);                       /* borrowed by open_ro_fd (dup'd internally) */
+        if (!v) return -1;                /* kcr3 no longer resolves the kernel => stale/guest-reboot */
+        /* Second, independent signal: the System process must be present AND its cr3 must equal
+         * the persisted kcr3 (the System DTB by definition). Catches the pathology "kcr3 resolves
+         * a DIFFERENT kernel". Cheap — the RO context is already built. Fail-closed on mismatch. */
+        process procs[16];
+        int n = proc_list(v, 0, procs, 16);
+        int system_ok = 0;
+        for (int i = 0; i < n && i < 16; i++)
+            if (!strcmp(procs[i].name, "System")) { system_ok = (procs[i].cr3 == rq->cr3); break; }
+        vmie_win32_close(v);              /* validation-only: the read datum needs no held handle */
+        if (!system_ok) return -1;
+        rs->kcr3 = rq->cr3;               /* validated: publish the read datum (NOT a->kcr3) */
+        return 0;
+#else
+        return -1;   /* armed without the build flag: resume impossible -> cold bootstrap */
+#endif
+    }
+
     /* MC_JOB_BOOTSTRAP */
     if (a->stub) {
         /* test-only: fail the first fail_boots attempts to exercise the retry path
@@ -259,6 +380,45 @@ static void mc_kick_bootstrap(struct vmsig_adapter* a) {
     (void)vmsig_worker_submit(a->worker, &rq, sizeof rq);   /* full => drop (rare) */
 }
 
+/* Submit the fast-path RESUME (off-loop: open_ro_fd reads image pages, not on the loop thread).
+ * Carries the persisted kcr3 + the cfg low for vmie_win32_open_ro_fd. On miss/validation-fail the
+ * completion handler falls back to a cold bootstrap — the persist never replaces it. */
+static void mc_kick_resume(struct vmsig_adapter* a, uint64_t kcr3) {
+    mc_req rq;
+    memset(&rq, 0, sizeof rq);
+    rq.op = MC_JOB_RESUME; rq.cr3 = kcr3; rq.low = a->low;
+    (void)vmsig_worker_submit(a->worker, &rq, sizeof rq);   /* full => drop (rare) */
+}
+
+/* Single publication path for BOTH RESUME and BOOTSTRAP (no two ways to publish a MEMCTX).
+ * Assembles the single-low locator from `kcr3` + a->low, marks have_ctx, and emits the MEMCTX
+ * trigger; the core authoritatively re-describes and stamps the epoch. Loop-thread-only.
+ *
+ * Ownership: this writes kcr3 ONLY into cur_pod.kcr3 (the delivery copy). It does NOT touch
+ * a->kcr3 — that is the gva_write TARGET, owned solely by the bootstrap worker. The difference
+ * between the two callers is only the SOURCE of kcr3 and whether an RW-hold / persist-save
+ * follows; the locator assembly itself is shared here. */
+static void mc_publish_ctx(struct vmsig_adapter* a, uint64_t kcr3) {
+    memset(&a->cur_pod, 0, sizeof a->cur_pod);
+    a->cur_pod.kcr3  = kcr3;
+    a->cur_pod.low   = a->low ? a->low : MC_STUB_SIZE;
+    a->cur_pod.flags = VMSIG_MEMCTX_RDONLY;
+    a->cur_nseg = 1;                       /* single-low identity (gpa 0 .. low) */
+    a->cur_segs[0].gpa = 0;
+    a->cur_segs[0].len = a->cur_pod.low;
+    a->cur_segs[0].file_off = 0;
+    a->cur_pod.nseg = a->cur_nseg;
+    a->have_ctx = 1;
+
+    /* emit the MEMCTX trigger: the core authoritatively re-describes + stamps the epoch. */
+    vmsig_event up;
+    memset(&up, 0, sizeof up);
+    up.kind = VMSIG_EV_MEMCTX; up.source = VMSIG_SRC_MEMCTX; up.dir = VMSIG_DIR_UP;
+    up.prio = VMSIG_PRIO_NORMAL; up.endpoint = a->endpoint;
+    memcpy(up.inln, &a->cur_pod, sizeof a->cur_pod);
+    a->emit.emit(a->emit.token, &up);
+}
+
 /* ---- reg hooks (vmsig_memctx_reg.ctx = a; called by the core on the loop thread) ---- */
 static void mc_reg_describe(void* ctx, vmsig_memctx* out_pod,
                             const vmsig_memseg** out_segs, uint32_t* out_nseg) {
@@ -286,6 +446,10 @@ static void mc_reg_invalidate(void* ctx, uint32_t epoch) {
     struct vmsig_adapter* a = ctx;
     (void)epoch;                     /* the core owns the epoch; the adapter must re-bootstrap */
     a->have_ctx = 0;                 /* the previous context is invalid */
+    /* destructive VM-lifecycle => the RAM may have changed => drop the kcr3 cache so the next
+     * restart cannot fast-path off a now-dead kcr3 (the self-validation would reject it anyway,
+     * but we do not leave a known-stale file). Best-effort, loop-thread-only. */
+    mc_persist_drop(a->persist_path);
     /* new cycle: drop a stale arm from the previous cycle and restart the failure counter at
      * zero so this bootstrap's backoff starts fresh (and the first-failure diagnostic re-arms). */
     a->boot_attempts = 0;
@@ -307,6 +471,7 @@ static vmsig_adapter* mc_open(const void* cfg, uint32_t endpoint) {
     a->stub_fd   = -1;
     a->retry_fd  = -1;
     a->fail_boots = c ? c->fail_boots : 0;   /* set once; read-only afterwards (worker reads) */
+    a->persist_path = c ? c->persist_path : NULL;   /* NULL => persist disabled (cold bootstrap only) */
     return a;
 }
 
@@ -364,7 +529,16 @@ static int mc_attach(vmsig_adapter* a, const vmsig_emit* emit, vmsig_fd_reg* reg
     up.prio = VMSIG_PRIO_NORMAL; up.endpoint = a->endpoint;
     a->emit.emit(a->emit.token, &up);
 
-    mc_kick_bootstrap(a);   /* first bootstrap off-loop; assemble the locator on completion */
+    /* Fast-path: if a kcr3 cache exists, try a RESUME (re-validate it against live RAM) BEFORE
+     * the cold scan. On a daemon restart over an unchanged guest this publishes the read datum
+     * in milliseconds instead of minutes of beacon-scan retry. On any miss (persist off / stub /
+     * no file / corrupt) we fall straight into the existing cold bootstrap. The RW-hold for
+     * MEMWRITE is still acquired by a cold bootstrap (kicked in parallel after a RESUME hit). */
+    mc_persist_blob b;
+    if (a->persist_path && *a->persist_path && mc_persist_load(a->persist_path, &b))
+        mc_kick_resume(a, b.kcr3);   /* validate the cached kcr3 off-loop; cold fallback on miss */
+    else
+        mc_kick_bootstrap(a);        /* first cold bootstrap off-loop; assemble locator on completion */
     return 2;                /* worker eventfd + backoff timerfd */
 }
 
@@ -391,6 +565,27 @@ static int mc_on_ready(vmsig_adapter* a, uint32_t cookie, uint32_t events) {
             mc_memwrite_ack(a, rs.ok && rc == 0, rs.corr, rs.origin);
             continue;
         }
+
+        if (rs.op == MC_JOB_RESUME) {
+            /* Fast-path completion. The persisted kcr3 was validated against the LIVE RAM on the
+             * worker (open_ro_fd != NULL [+ System-cr3 match]) — the read datum is safe to publish.
+             * Note: the worker did NOT set a->kcr3/a->win/a->mem (the RW write-hold), so MEMWRITE
+             * stays ok=0 until a cold bootstrap acquires it. */
+            if (rc == 0) {
+                mc_publish_ctx(a, rs.kcr3);   /* video lives instantly (read datum), epoch by core */
+                mc_kick_bootstrap(a);         /* in parallel: acquire the RW-hold (a->kcr3) for MEMWRITE */
+                /* Do NOT save the persist (the kcr3 came FROM the file) and do NOT arm a retry
+                 * (the read datum is up; the parallel bootstrap arms its own retry on failure). */
+            } else {
+                /* validation miss: the persisted kcr3 no longer resolves the kernel (guest rebooted
+                 * or corrupt). Fall back to an honest cold scan; on success it rewrites the persist
+                 * with a fresh kcr3. Do NOT retry the RESUME — the cache is under suspicion. */
+                mc_kick_bootstrap(a);
+            }
+            continue;
+        }
+
+        /* MC_JOB_BOOTSTRAP */
         if (rc != 0) {
             /* bootstrap failed: the guest is likely still booting (host_bootstrap found no
              * System process). This is NOT a control-level error — do NOT emit VMSIG_EV_ERROR
@@ -405,34 +600,29 @@ static int mc_on_ready(vmsig_adapter* a, uint32_t cookie, uint32_t events) {
             mc_arm_retry(a);   /* one-shot timer at mc_boot_backoff(boot_attempts) */
             continue;
         }
-        /* assemble the locator on the loop thread from rs.kcr3. a->kcr3 is the gva_write
+        /* bootstrap succeeded: a->kcr3/a->mem (the gva_write TARGET / RW-hold) were set on the
-         * TARGET and is owned SOLELY by the worker thread (set in mc_bootstrap_armed, read by
+         * worker (mc_bootstrap_armed); the loop must NOT also write a->kcr3 (it would race an
-         * MC_JOB_WRITE — same thread, FIFO happens-before); the loop must NOT also write it, or
+         * in-flight write — same FIFO thread owns it). MEMWRITE is now possible. cur_pod.kcr3 is
-         * an in-flight write at line ~170 would race it. cur_pod.kcr3 is loop-only (delivery). */
+         * loop-only (delivery) and is set inside mc_publish_ctx.
-
+         *
-        /* bootstrap succeeded: cancel any pending retry and reset the failure counter BEFORE
+         * Cancel any pending retry and reset the failure counter BEFORE publishing, so a stale
-         * publishing, so a stale timer armed by a prior failure cannot fire over a live context. */
+         * timer armed by a prior failure cannot fire over a live context. */
         a->boot_attempts = 0;
         mc_disarm_retry(a);
 
-        memset(&a->cur_pod, 0, sizeof a->cur_pod);
+        /* Publish only if a RESUME has not already published this same context (same kcr3): a
-        a->cur_pod.kcr3  = rs.kcr3;
+         * parallel cold bootstrap after a RESUME hit must acquire the RW-hold WITHOUT emitting a
-        a->cur_pod.low   = a->low ? a->low : MC_STUB_SIZE;
+         * redundant MEMCTX. First-time publication otherwise. */
-        a->cur_pod.flags = VMSIG_MEMCTX_RDONLY;
+        if (!a->have_ctx)
-        a->cur_nseg = 1;                       /* single-low identity (gpa 0 .. low) */
+            mc_publish_ctx(a, rs.kcr3);
-        a->cur_segs[0].gpa = 0;
-        a->cur_segs[0].len = a->cur_pod.low;
-        a->cur_segs[0].file_off = 0;
-        a->cur_pod.nseg = a->cur_nseg;
-        a->have_ctx = 1;
 
-        /* emit the MEMCTX trigger: the core authoritatively re-describes + stamps the epoch. */
+        /* Cache the freshly-scanned kcr3 for the next daemon restart (best-effort; the datum is
-        vmsig_event up;
+         * already published). Only the cold scan writes the persist — never the RESUME path (its
-        memset(&up, 0, sizeof up);
+         * kcr3 came from the file). Gated on persist_path presence: production stub paths get a
-        up.kind = VMSIG_EV_MEMCTX; up.source = VMSIG_SRC_MEMCTX; up.dir = VMSIG_DIR_UP;
+         * NULL persist_path from discovery, so they never write; a test may supply one to exercise
-        up.prio = VMSIG_PRIO_NORMAL; up.endpoint = a->endpoint;
+         * the persist mechanics (the stub bootstrap yields a synthetic-but-stable kcr3). */
-        memcpy(up.inln, &a->cur_pod, sizeof a->cur_pod);
+        if (a->persist_path && *a->persist_path)
-        a->emit.emit(a->emit.token, &up);
+            (void)mc_persist_save(a->persist_path, rs.kcr3);
     }
     return 0;
 }

src/discovery/discovery.c

@@ -60,6 +60,10 @@ struct vmsig_discovery {
      * writes these at attach; the vmhost seam borrows them to add input-linux objects. Same
      * lifetime discipline as ep_facts (outlives the deferred adapter reap). */
     struct { char evdev_a[64]; char evdev_b[64]; } ep_bridge[VMSIG_SLOT_COUNT];
+    /* Stable per-endpoint home for the memctx kcr3-cache path (sibling of .slots in the watch
+     * dir). The memctx adapter keeps the pointer across its lifetime; same lifetime discipline
+     * as ep_facts/ep_bridge (outlives the deferred adapter reap, overwritten on next attach). */
+    char                 ep_persist[VMSIG_SLOT_COUNT][DISC_PATH_MAX + 32];
 };
 
 static uint64_t now_ns(void) {
@@ -269,14 +273,25 @@ static void bootstrap_scan(vmsig_discovery* d) {
 
 static int default_attach(void* ud, vmsig_core* core, uint32_t vmid, uint32_t endpoint,
                           const vmsig_host_facts* f) {
-    (void)vmid;
     vmsig_discovery* d = ud;   /* default sink carries the discovery handle (ep_bridge home) */
     char* ev_a = d ? d->ep_bridge[endpoint].evdev_a : NULL;
     char* ev_b = d ? d->ep_bridge[endpoint].evdev_b : NULL;
     if (d) { ev_a[0] = '\0'; ev_b[0] = '\0'; }   /* clear stale paths from a prior attach */
 
+    /* Form the kcr3-cache path (per-vmid, sibling of .slots/the RAM file in the watch dir).
+     * Gated on d->persist — one policy for all ephemeral watch-dir state. NULL => persist off. */
+    const char* persist_path = NULL;
+    if (d && d->persist) {
+        int pn = snprintf(d->ep_persist[endpoint], sizeof d->ep_persist[endpoint],
+                          "%s/.kcr3-vm-%u", d->watch_dir, vmid);
+        /* only enable the cache if the path fit (a truncated path would point elsewhere). */
+        if (pn > 0 && (size_t)pn < sizeof d->ep_persist[endpoint])
+            persist_path = d->ep_persist[endpoint];
+    }
+
     vmsig_memctx_cfg mc; memset(&mc, 0, sizeof mc);
     mc.stub = 0; mc.ram_path = f->ram_path; mc.low = f->low; mc.ro_fd = -1;
+    mc.persist_path = persist_path;
     vmsig_input_cfg in; memset(&in, 0, sizeof in);
     /* input is uinput; power/lifecycle via the vmhost seam. The adapter publishes its uinput
      * evdev paths into ep_bridge so the vmhost seam can forward them via input-linux. */

src/test/test_memctx.c

@@ -428,6 +428,130 @@ static void test_retry(void) {
     vmsig_ctx_free(ctx);
 }
 
+/* ---- 8-11. kcr3-persist MECHANICS (stub) ---------------------------------- *
+ * These exercise the persist MACHINERY only: save/load, corruption fail-soft, drop-on-
+ * invalidate, and the fast-vs-slow path selection. They do NOT exercise the real boot-session
+ * validation (vmie_win32_open_ro_fd rejecting a stale kcr3) — that is VMIE-dependent and is
+ * covered only on the armed stand. Under the stub, MC_JOB_RESUME synthetically ACCEPTS any
+ * nonzero kcr3 (there is no live RAM to validate against), so a successful RESUME here proves
+ * the mechanism wired the cached kcr3 into a publication, NOT that the kcr3 was validated. */
+
+static int file_exists(const char* path) { return access(path, F_OK) == 0; }
+
+/* Run a memctx endpoint to its first MEMCTX (or the ticks failsafe) over a private core. */
+static void run_once(uint64_t* out_kcr3, int* out_memctx, const char* persist_path,
+                     uint32_t fail_boots) {
+    vmsig_ctx*  ctx  = vmsig_ctx_new();
+    vmsig_core* core = vmsig_core_new(ctx);
+
+    holder h; memset(&h, 0, sizeof h);
+    h.core = core; h.is_driver = 1; h.expect_ep = 0; h.stop_epoch = -1;
+    add_holder(core, &h, VMSIG_CAP_MEMCTX | VMSIG_CAP_OBSERVE, 0xFFFFFFFFu, 1ull << 0);
+
+    CHECK(vmsig_core_add_adapter(core, vmsig_vmhost_ops(), NULL, 0) >= 0, "add vmhost (watchdog)");
+
+    vmsig_memctx_cfg mc; memset(&mc, 0, sizeof mc);
+    mc.stub = 1; mc.ram_path = NULL; mc.low = 0; mc.ro_fd = -1;
+    mc.fail_boots = fail_boots; mc.persist_path = persist_path;
+    CHECK(vmsig_core_add_adapter(core, vmsig_memctx_ops(), &mc, 0) >= 0, "add memctx");
+
+    vmsig_core_run(core);
+    if (out_kcr3)   *out_kcr3   = h.last_kcr3;
+    if (out_memctx) *out_memctx = h.memctx;
+
+    vmsig_core_free(core);
+    vmsig_ctx_free(ctx);
+}
+
+/* 8. save-then-resume: run1 (cold stub bootstrap) publishes MEMCTX and WRITES the cache; run2
+ * over the SAME persist_path takes the RESUME fast-path. The KEY is fail_boots=large in run2:
+ * if it had gone through a cold bootstrap it would have failed N times (no MEMCTX inside the
+ * loop budget); a prompt MEMCTX carrying the SAVED kcr3 proves RESUME bypassed the bootstrap. */
+static void test_persist_save_then_resume(void) {
+    printf("test_persist_save_then_resume\n");
+    char path[256];
+    snprintf(path, sizeof path, "/tmp/vmsig-kcrx-%d.bin", (int)getpid());
+    unlink(path);
+
+    uint64_t k1 = 0; int m1 = 0;
+    run_once(&k1, &m1, path, 0);
+    CHECK(m1 >= 1, "run1 published MEMCTX");
+    CHECK(k1 != 0, "run1 kcr3 nonzero");
+    CHECK(file_exists(path), "run1 wrote the kcr3 cache file");
+
+    /* run2: a cold bootstrap would fail 1000 times — only RESUME can publish promptly. */
+    uint64_t k2 = 0; int m2 = 0;
+    run_once(&k2, &m2, path, 1000);
+    CHECK(m2 >= 1, "run2 published MEMCTX via the RESUME fast-path (bootstrap would have failed)");
+    CHECK(k2 == k1, "run2 published the SAVED kcr3 (resumed from cache, not a fresh scan)");
+
+    unlink(path);
+}
+
+/* 9. corrupt file => load fail-soft => cold bootstrap still brings the context up. */
+static void test_persist_corrupt(void) {
+    printf("test_persist_corrupt\n");
+    char path[256];
+    snprintf(path, sizeof path, "/tmp/vmsig-kcrx-corrupt-%d.bin", (int)getpid());
+    int fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0600);
+    CHECK(fd >= 0, "created a corrupt cache file");
+    if (fd >= 0) { (void)!write(fd, "x", 1); close(fd); }   /* 1 byte: short/wrong magic */
+
+    uint64_t k = 0; int m = 0;
+    run_once(&k, &m, path, 0);   /* load miss => cold bootstrap (fail_boots=0 => succeeds) */
+    CHECK(m >= 1, "MEMCTX still published after a corrupt cache (fail-soft load)");
+    CHECK(k != 0, "kcr3 nonzero from the cold bootstrap");
+
+    unlink(path);
+}
+
+/* 10. invalidate drops the cache; the re-bootstrap on the new epoch rewrites it fresh. */
+static void test_persist_invalidate_drop(void) {
+    printf("test_persist_invalidate_drop\n");
+    char path[256];
+    snprintf(path, sizeof path, "/tmp/vmsig-kcrx-inv-%d.bin", (int)getpid());
+    unlink(path);
+
+    vmsig_ctx*  ctx  = vmsig_ctx_new();
+    vmsig_core* core = vmsig_core_new(ctx);
+
+    holder h; memset(&h, 0, sizeof h);
+    /* inject a destructive lifecycle on epoch0 (as test_epoch); stop after epoch1. */
+    h.core = core; h.is_driver = 1; h.expect_ep = 0; h.inject_reset = 1; h.stop_epoch = 1;
+    add_holder(core, &h, VMSIG_CAP_MEMCTX | VMSIG_CAP_OBSERVE, 0xFFFFFFFFu, 1ull << 0);
+
+    CHECK(vmsig_core_add_adapter(core, vmsig_vmhost_ops(), NULL, 0) >= 0, "add vmhost (watchdog)");
+    vmsig_memctx_cfg mc; memset(&mc, 0, sizeof mc);
+    mc.stub = 1; mc.ram_path = NULL; mc.low = 0; mc.ro_fd = -1; mc.persist_path = path;
+    CHECK(vmsig_core_add_adapter(core, vmsig_memctx_ops(), &mc, 0) >= 0, "add memctx");
+
+    vmsig_core_run(core);
+
+    /* epoch0 bootstrap wrote the cache; invalidate dropped it; epoch1 bootstrap rewrote it. */
+    CHECK(h.invalidated >= 1, "invalidation fired");
+    CHECK(h.last_epoch == 1,  "re-published at epoch 1 after invalidate");
+    CHECK(file_exists(path),  "cache rewritten by the post-invalidate bootstrap");
+
+    vmsig_core_free(core);
+    vmsig_ctx_free(ctx);
+    unlink(path);
+}
+
+/* 11. persist disabled (persist_path=NULL): no cache file is ever created (today's behavior). */
+static void test_persist_stub_disabled(void) {
+    printf("test_persist_stub_disabled\n");
+    char path[256];
+    snprintf(path, sizeof path, "/tmp/vmsig-kcrx-off-%d.bin", (int)getpid());
+    unlink(path);
+
+    uint64_t k = 0; int m = 0;
+    run_once(&k, &m, NULL, 0);   /* persist off */
+    CHECK(m >= 1, "MEMCTX published with persist disabled");
+    CHECK(!file_exists(path), "no cache file created when persist is disabled");
+
+    unlink(path);   /* belt-and-braces */
+}
+
 int main(void) {
     test_multicast();
     test_epoch();
@@ -436,6 +560,10 @@ int main(void) {
     test_socket();
     test_ro_fd_ownership();
     test_retry();
+    test_persist_save_then_resume();
+    test_persist_corrupt();
+    test_persist_invalidate_drop();
+    test_persist_stub_disabled();
     printf("memctx tests: %s\n", g_fail ? "FAIL" : "PASS");
     return g_fail ? 1 : 0;
 }