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

kernel/dev/pci.c

@@ -77,6 +77,24 @@ pci_enable(struct pci_dev *d, uint16 bits)
   pci_write32(d->bus, d->slot, d->func, PCI_COMMAND, cmd | bits);
 }
 
+// The other direction, and PCI_CMD_BUS_MASTER is the reason it exists.
+//
+// Clearing that bit is the only way this kernel can stop a device it does not
+// understand from touching memory. Everything else about halting a device is
+// device-specific -- a different register on every chip -- but bus mastering is
+// in the standard config header of every PCI device ever made, and clearing it
+// means the next DMA cycle does not happen.
+//
+// That makes it the correct thing to do when a userspace driver dies: the
+// kernel is about to take its DMA buffers back, and a device still scanning out
+// of them would read whatever is put there next. See kernel/dev/pciuser.c.
+void
+pci_disable(struct pci_dev *d, uint16 bits)
+{
+  uint32 cmd = pci_read32(d->bus, d->slot, d->func, PCI_COMMAND);
+  pci_write32(d->bus, d->slot, d->func, PCI_COMMAND, cmd & ~(uint32)bits);
+}
+
 // ---------------------------------------------------------------------------
 // Capabilities
 //