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/hybrid.md mechanism 3. A process asks for a device's interrupts with
PCIIOC_IRQ and then reads them, which is how Linux's uio does it and for the
same reason: a descriptor composes with poll, and a driver that stops reading
falls behind rather than losing events. read() returns a count rather than a
flag, so a driver that was busy sees it missed three and acts once.
Proven against an e1000e by asserting a cause through its ICS register and
reading the interrupt back out in userspace:
interrupts routed here (line 10)
interrupt delivered, count = 1
ICR = 0x80000004
0x80000004 is the cause the test set plus INT_ASSERTED -- the device's own
register, read by userspace as its acknowledgement.
**The masking is the design, not a detail.** A PCI line is level-triggered and
stays asserted until the cause is cleared at the device, which is a different
register on every chip and so is something only the driver can do. The kernel
masks the line on arrival and userspace's write() unmasks it. Signalling EOI on
a still-asserted line re-enters the handler immediately and for ever; this
kernel did exactly that by accident at 72,000 interrupts a second, and what it
looked like from outside was a clock running seven times slow (§13).
Which decides what happens when a driver dies mid-interrupt: the line is left
**masked**. The instinct is to unmask -- do not leave the device dead -- and it
is wrong, because unmasking an asserted line with no handler is a crashed
driver taking the machine down with it. Masked is safe and not permanent: the
next PCIIOC_IRQ arms it. Both halves verified by crashing a driver on purpose
and then starting another one.
A line already carrying kernel handlers is refused outright. Masking is
per-line, so handing out a shared one would silence whichever kernel driver is
also there, with nothing logged. MSI is the real answer and pci_msi_enable() is
already in the tree waiting for a caller.
Also fixed, found by running the crash twice: DMA buffers were never returned
when a process exited. The arena is 4 MB and contiguous, so a driver that
restarts a few times exhausted it -- "out of DMA memory, 3584 free" -- which is
a poor property for an interface whose whole promise is that a driver can crash
and come back.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
docs/plan/hybrid.md mechanisms 2 and 4: MMIO and DMA to a process.
A process can enumerate the bus, claim a device exclusively, map its BARs and
get memory the device can be pointed at. Proven end to end against an e1000e
-- the exact device class we cannot drive and would want a foreign driver for:
device 7: 8086:10d3 (network)
claimed
BAR0 at 0x81060000, 128 KB
regs[0..3] = 0x00140261 0x00000000 0x00080283 0x00000000
MMIO reaches the device
DMA 8 KB at virtual 0x40001000, physical 0x3f8eb000
Those are the e1000e's CTRL and STATUS with full-duplex and link-up set, which
is how you tell a real mapping from one reading 0xFFFFFFFF off an unmapped
address. Claiming a device the kernel drives is refused -- two writers of one
register set looks like hardware that has started lying -- and everything a
process held is released when it exits, because a driver that crashed must not
keep its hardware away from its replacement.
mmap's offset is the BAR's physical address, checked against the BARs of
devices *this process has claimed*. Without that check the device is /dev/mem
with extra steps and a process could map the kernel's own memory. Mappings are
write-through: a cached write to a doorbell arrives whenever the cache feels
like it, which from a driver's side is indistinguishable from hardware that
ignores you.
The interface is about PCI devices, not about any driver model, and its first
users will be SchOS's own drivers when virtio-gpu is evicted. That ordering is
the design rather than an accident: an interface whose first consumers are
native drivers is a general mechanism something foreign can use later, instead
of a hole shaped like something foreign.
Also: PROT_* and MAP_* moved to include/posix.h. They were defined only in
sys_mem.c, so cmd/fbinfo carried a private copy and this wanted a third --
two copies of a number the kernel compares against, where a mismatch shows up
as a mapping with the wrong permissions rather than an error.
No IOMMU, so this isolates a driver against its own bugs and not against its
own DMA. The plan says so at length and it is still true.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
Nexoniarz