# session 77 (r683/r684) — vkCreateDevice PASSES: the 0x7E event validation and the readiness-word completion

## The two-part unblock

1. **r682 fix validated** (`e3c1cdc5`, loaded on r683/r684): the
   nvkms/Vulkan path allocates its completion event as
   NV01_EVENT_KERNEL_CALLBACK_EX (class 0x7E); the emulation now
   registers it alongside 0x79. Live proof (r683, sema_dbg=1):

       GSP_RM_ALLOC class=0x7e paramsSize=24 hClient=0xc1e00011
           hParent=0xcaf00001 hObject=0xcaf00003
       event registered hEvent=0xcaf00003 hSrc=0x0
           notifyIndex=1006632972 (off 8, slot 13)
           params=1100e0c1000000007e0000000c00003c0000000000000000
       EVPOST all ev[13] cl=0xc1e00011 ev=0xcaf00003 nidx=0x3c00000c rc=0

   (notifyIndex 0x3c00000c = subdevice 0x3c, index 0xc =
   NV2080_NOTIFIERS_GRAPHICS). The guest consumes the records
   (`guest_read == our_write` in the rmstate dump), but the guest's
   event dispatch does NOT write the event's notification word — the
   residual delivery gap.

2. **The completion word found and driven (the actual unblock)**. The
   main-thread vkCreateDevice wait (eglcore 0xdceb80, session 76) reads
   a u32 readiness word: `should-wait = (word != 1)`; the word lives at
   `+0x80` of a **16 KB rw-s /dev/nvidiactl mapping** (the event
   notification buffer; holder chain sub -> obj2 -> holder, resolved
   live by t79.py). The host can write it directly in the guest RAM
   file:

       PA = pagemap PFN << 12  (+0x80);  file offset = PA - 0x80000000
       (above-4G RAM; below-4G would be PA itself per the cycle's
       guest-RAM registration: gpa 0x0/0x100000000 -> off 0x0/0x80000000)

   Writing 1 flipped the wait and **vkCreateDevice returned success**
   (`VKDEVICE ok family=0`) — reproduced three times:
   * r683: manual write after a t79 probe;
   * r684: nudger (all 16 KB mapping words) + slotfill restart;
   * r684d: probe -> single-word write (the reliable recipe).

## The reliable recipe (per run)

1. r682+ module; `sema_dbg=1 gsp_rm_ev_log=300` optional.
2. Launch `vk_frame` + `slotfill2.sh` (EGL completion-slot fills; the
   slots re-arm, so re-run slotfill if the client is stuck in the
   sched_yield spin before the word-wait).
3. Wait for the main thread in `do_poll` (the word-wait).
4. `t79.py` (gdb) -> `WAIT word=0x...`; guest pagemap -> PFN -> PA;
   host write `01 00 00 00` at `PA-0x80000000`.
5. `VKDEVICE ok` follows within ~10 s. The client then proceeds to
   image/memory/pipeline setup; its next wait is NOT the same word
   (the word stays 1) — next session: probe the follow-on state
   (attach fails while the client spins in the post-device phase;
   use the /proc-only checks + a longer observation window).

## Cautions learned

* Writing ALL 16 KB mapping words (the multi-page nudger) can trip the
  -4 give-up path (an RM error flag word gets written): restrict nudges
  to the single probed word.
* Attaching gdb to the client while it is in the tight post-device spin
  can hang the attach (leave a `timeout`-wrapped gdb, then kill that
  gdb by PID and `kill -CONT` the client — the pending SIGSTOP from a
  dead tracer otherwise wedges it).
* zsh truncates 64-bit hex literals in `$(( ))`: compute PFN/PA in
  python, not zsh.

## Tools

* `t79.py` — resolves the wait chain (sub -> obj2 -> holder -> word VA)
  and prints the client's nvidiactl rw-s mapping sizes.
* `guest_words.sh` — prints VA/PA of the +0x80 word for every 16 KB
  nvidiactl mapping (pagemap; run in the guest as root).
* `nudge.sh` — host-side loop: poll guest_words.sh and write 1 to any
  zero word (multi-page variant; prefer the single-word recipe).