hybrid: the driver is killed, and the driver comes back

H2 of docs/plan/hybrid.md -- the milestone the plan itself calls the one
that proves the whole idea, and the one that cannot be skipped.

    # cat /proc/schinit
    1 4 11 os.schos.vgpud
    # kill 4
    schinitd: restarting os.schos.vgpud
    vgpud: claimed
    vgpud: pid 8 drew a test pattern from userspace

The test pattern ends in a twelve-block tally of the drawing process's
pid, because a second run that redraws an identical picture is
indistinguishable from a first run that never stopped. Read off the
virtio-gpu scanout across four generations it says 4, 8, 10, 12, and the
DMA arena hands back the same physical address every time.

What it took:

* A halt the kernel can apply to a device it does not understand.
  Clearing PCI_CMD_BUS_MASTER is the only one there is, and it has to
  happen before the DMA goes back to the arena -- a dead driver's device
  keeps reading a framebuffer the kernel is about to give to somebody
  else. Claim sets the bit, release clears it; pcitest shows it in a bm
  column, with a third state so a failed read is never printed as off.

* A refusal. vgpu_release() asks vga_fb_restore() whether there is a
  framebuffer to put the console back on, and declines to hand over the
  device if there is not. The rule that the kernel keeps a minimal
  driver for everything on the path to a shell is worth writing down
  only if something enforces it.

* The DMA arena not being owned by two allocators at once. This is the
  bug that stopped H2: freevm() frees every mapped page below PHYSTOP
  and the arena is the top 16 MB of exactly that range, so a DMA block
  mapped into a driver went onto the page allocator's free list when the
  driver died, while pciuser_release_all() handed the same pages back to
  the arena. The restarted vgpud asked for its framebuffer, got pages
  kalloc() had already given out -- including its own page tables -- and
  dma_alloc()'s zeroing wiped them, so it died inside the ioctl on the
  return from a syscall whose stack had stopped existing. do_dma() now
  records a VMA with no_free set, the way shared memory does.

* One exit status. KeepAlive must restart a driver that crashed and must
  not restart one that has looked at the machine and found its device
  absent: on hardware with no virtio-gpu that was fifteen spawns in
  forty-five seconds, each printing a line to the console, for ever.
  SCHINIT_EXIT_NORESTART is 78, sysexits.h's EX_CONFIG. It is not a new
  manifest key -- the wait status has always travelled from PID 1 to the
  supervisor, and note_exit() simply stopped discarding it.

The milestone's third clause, "the compositor reattaches", is not done
and is not claimed: there is no compositor running. That is recorded in
the plan as a Schcom milestone rather than counted here.

Closes the register row "A released PCI device" in docs/plan/1.0.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

docs/plan/1.0.md

@@ -94,9 +94,14 @@ The galling ones. The work exists, is tested, and nothing uses it.
 | | Where it stopped |
 |---|---|
 | **`foundations.md`** | Item 1 built. Items 2 (a shareable memory object), 3 (`AF_UNIX` in the filesystem namespace), 4 and 5 open. **Wayland stops at item 2.** |
-| **`hybrid.md`** | All four mechanisms built, H1 done. **H2 — kill the driver on purpose and watch it come back — is the milestone the plan itself calls the one that proves the whole idea**, and it is not done. |
+| **`hybrid.md`** | All four mechanisms built. H1 and H2 done — the driver can be killed and comes back, which is the milestone the plan calls the one that proves the whole idea. H3 (a modeset API) and H4 (USB) open. |
 | **Schcom v0** | 715 lines that composite, over a wire format that is superseded. Frozen: it runs, nothing more is added, and it is kept only because it is the one thing in the tree that has ever put a client's pixels on screen. |
-| **A released PCI device** | `PCIIOC_RELEASE` leaves the line masked and the in-kernel driver unbound. Nothing starts a replacement. Half a handover. |
+
+**Closed since this register was written.** *A released PCI device* — release is
+now symmetric with claim (bus mastering cleared before the DMA is freed, a
+refusal when the device is the only display), a supervised job starts the
+replacement, and it is described in
+[../architecture/devices.md](../architecture/devices.md) under "Taking it back".
 
 ### Named in a "what it does not do" and still true
 

docs/plan/hybrid.md

@@ -172,6 +172,79 @@ version that survives a licensing conversation.
 
 ---
 
+## The driver came back
+
+H2 is the milestone this plan calls the one that cannot be skipped, and the
+reason is that H1 proves only half of the idea. A driver that runs in a process
+is interesting; a driver that runs in a process, *dies*, and is replaced by
+another process that picks the device up is the thing the whole hybrid argument
+rests on.
+
+`etc/schinit/vgpud.job` makes it a supervised job -- `KeepAlive = yes` -- so
+killing it is the whole test:
+
+```
+# cat /proc/schinit
+1 4 11 os.schos.vgpud
+# kill 4
+schinitd: restarting os.schos.vgpud
+vgpud: virtio-gpu at 00:03.0
+vgpud: claimed
+vgpud: display is 1280x800
+vgpud: scanout is ours, framebuffer at physical 0x3eceb000
+vgpud: pid 8 drew a test pattern from userspace
+```
+
+The test pattern ends in a twelve-block tally of the drawing process's pid,
+because a second run that redraws an identical picture is indistinguishable
+from a first run that never stopped. Read off the virtio-gpu scanout across
+four generations of the driver it says 4, 8, 10, 12. The same physical address
+comes back every time, which is the DMA arena reclaiming cleanly rather than
+drifting.
+
+### What it took that H1 had not needed
+
+- **A halt the kernel can apply without knowing what the device is.** A driver
+  that dies leaves its device bus-mastering into memory the kernel is about to
+  hand to somebody else. The kernel cannot quiesce a device it does not
+  understand, but it can clear `PCI_CMD_BUS_MASTER`, and that is enough to stop
+  the writes. `pciuser_stop()` does it before the DMA is freed, not after, and
+  claim/release became a symmetric pair: claiming sets the bit, releasing
+  clears it. `pcitest` grew a `bm` column so the state is visible rather than
+  assumed.
+- **A refusal, when there is nothing to fall back to.** `vgpu_release()` now
+  asks `vga_fb_restore()` whether there is a framebuffer to put the console on,
+  and declines to hand the device over if there is not. The rule that the
+  kernel keeps a minimal driver for everything on the path to a shell is only
+  worth writing down if something enforces it.
+- **The DMA arena not being handed to two allocators at once.** This is the bug
+  that stopped H2 for a day, and it is worth writing out: `freevm()` frees
+  every mapped page below `PHYSTOP`, and the DMA arena is the top 16 MB of
+  exactly that range. So a DMA block mapped into a driver and left for
+  `freevm()` went onto the page allocator's free list when the driver died,
+  while `pciuser_release_all()` handed the same pages back to the arena. Both
+  allocators then believed they owned it. The restarted `vgpud` asked for its
+  4 MB framebuffer, got pages `kalloc()` had already given out -- including its
+  own page tables -- and `dma_alloc()`'s zeroing wiped them, so the driver died
+  inside the ioctl, on the return from a syscall whose stack had stopped
+  existing. `do_dma()` now records a VMA with `no_free` set, the same way
+  shared memory does, so `proc_release_shared()` unmaps the range before the
+  page tables are torn down.
+
+### The clause that is not true yet
+
+The milestone says *"the compositor reattaches"*. It does not, because there is
+no compositor running: Schcom is v0 and is not started at boot. What has been
+shown is the two parts that do not depend on it -- the screen falls back to
+VBE, and the driver restarts and takes the device back. During the gap the VBE
+head carries a live console and the virtio-gpu head still shows the dead
+driver's last frame, which is exactly what a device nobody is driving does.
+
+Reattachment is a Schcom milestone, not a hybrid one, and it is recorded that
+way rather than counted here.
+
+---
+
 ## The mechanisms
 
 Four, and only the fourth is large.
@@ -279,13 +352,13 @@ start because the display driver is a process that needs a compositor.
 
 ## Milestones
 
-| | |
-|---|---|
-| **H0** | Mechanisms 2, 3 and 4, each with a test that exercises it from a userspace program against a device nothing else is using. |
-| **H1** | A userspace virtio-gpu driver, publishing `display_ops` across the channel. Schcom talks to it and does not know the difference. |
-| **H2** | Kill the driver process on purpose. The screen falls back to VBE, the driver restarts, the compositor reattaches. **This is the milestone that proves the whole idea**, and it is the one that cannot be skipped. |
-| **H3** | A modeset API, which is the thing an out-of-kernel GPU driver makes affordable and an in-kernel one never got. |
-| **H4** | USB, same shape. |
+| | | |
+|---|---|---|
+| **H0** | Mechanisms 2, 3 and 4, each with a test that exercises it from a userspace program against a device nothing else is using. | done |
+| **H1** | A userspace virtio-gpu driver, publishing `display_ops` across the channel. Schcom talks to it and does not know the difference. | done, except that Schcom does not talk to it -- `cmd/vgpud` holds the device and draws its own frame |
+| **H2** | Kill the driver process on purpose. The screen falls back to VBE, the driver restarts, the compositor reattaches. **This is the milestone that proves the whole idea**, and it is the one that cannot be skipped. | done, less the compositor -- see "The driver came back" |
+| **H3** | A modeset API, which is the thing an out-of-kernel GPU driver makes affordable and an in-kernel one never got. | |
+| **H4** | USB, same shape. | |
 
 ---