125 lines
6.6 KiB
Markdown
125 lines
6.6 KiB
Markdown
# IPC: message-passing channels
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Inter-process communication is the **backbone of a microkernel**. Once drivers and
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services run isolated in their own address spaces ([vision](vision.md)), they can't
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just call each other — a request becomes a **message**. In a microkernel, whatever
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was a function call across a monolithic kernel is IPC, so it's a first-class
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concern, not an afterthought.
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There are two layers, built a milestone apart:
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- **`system/kernel/ipc.zig`** — a bounded blocking channel between *kernel threads*,
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described below. The primitive, and where the blocking discipline was worked out.
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- **`system/kernel/ipc-synchronous.zig`** — synchronous call/reply between *processes*, across
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address spaces. What user-space servers and drivers actually talk over. It's the
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second half of this document.
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## The channel
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The first form is a **bounded blocking channel** (`system/kernel/ipc.zig`): a fixed-size
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ring buffer of messages with a producer/consumer rendezvous, built on the
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scheduler's [wait queues](scheduling.md).
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`Channel(T, capacity)` is generic over the message type and buffer size. It holds a
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ring buffer, a count, and two wait queues:
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- **`send(msg)`** — if the channel is full, block on the *not-full* queue; otherwise
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write the message, bump the count, and wake a waiting receiver.
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- **`recv()`** — if the channel is empty, block on the *not-empty* queue; otherwise
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take a message, drop the count, and wake a waiting sender.
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Neither side busy-waits: a full channel parks the sender, an empty one parks the
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receiver, and each operation wakes the other side when it makes progress possible.
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Two details make it correct:
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- **Recheck in a loop.** A woken task re-tests the condition (`while (full) wait`)
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rather than assuming the slot is still available — another waiter may have taken
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it first. This is the standard guard against spurious or racing wakeups.
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- **One critical section.** `send`/`recv` run under `saveInterrupts` /
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`restoreInterrupts` (the composable form, see [scheduling.md](scheduling.md)), so
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checking the condition and committing the block/enqueue happen atomically with
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respect to the timer preempting mid-operation. `waitLocked` / `wakeLocked` are the
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variants that assume the caller already holds that critical section.
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## Verifying it
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The `ipc` test (see [testing.md](testing.md)) runs a producer and a consumer passing
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**100 messages through a 4-slot channel**. The small buffer means the channel goes
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full and empty over and over, so both the blocking-send and blocking-recv paths are
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exercised heavily. The messages arrive intact and in order (their sum is the
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expected `5050`), and neither task busy-waits — they block and wake each other.
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## Endpoints: call/reply across address spaces
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A channel connects two kernel threads sharing one address space. Real servers are
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*processes*, so the payload has to cross an address-space boundary. That's
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`system/kernel/ipc-synchronous.zig`, and its shape is L4's: a synchronous **rendezvous** at an
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`Endpoint`, with the message copied directly from the sender's pages to the receiver's
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(`copyAcross` walks both sets of page tables through the physmap — no CR3 switch, no
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bounce buffer).
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Two syscalls carry it:
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- **`ipc_call(h, msg, reply)`** — copy `msg` to the server, block until it replies.
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- **`ipc_reply_wait(h, reply, recv)`** — reply to the client you're still holding (if
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any), then block for the next request. One syscall, because a server's steady state
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is *always* "finish the last one, wait for the next".
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An endpoint is reached by **handle** — a small integer index into the process's handle
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table (`Task.handles`), exactly like a file descriptor, and just as unforgeable. The
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bootstrap problem (how do you get the first handle?) is solved by a tiny name registry:
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a server calls `ipc_register(service_id, h)` under a well-known small integer, and a
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client calls `ipc_lookup(service_id)`.
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The server never learns the client's identity beyond a **badge**, delivered alongside
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the message: the caller's task id.
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### Interrupts are messages too
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`notifyFromIsr` posts an *asynchronous* notification to an endpoint — no payload, no
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reply owed — and wakes whoever is blocked in `reply_wait`. Its badge has the top bit
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set (`notify_badge_bit`), which is how a driver's single event loop distinguishes "a
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client wants something" from "the hardware wants something". Notifications sit in a
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small coalescing ring on the endpoint, so an interrupt taken while the driver was busy
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elsewhere is not lost.
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This is what makes a user-space driver possible at all, and it's the subject of
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[drivers.md](drivers.md).
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## What's next (not done here)
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- **Priority inheritance** through IPC, so a high-priority client blocked on a
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low-priority server doesn't suffer unbounded priority inversion.
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- **Handle transfer.** A server can't hand a client a handle to a third endpoint, so
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every capability is either well-known (the registry) or inherited — there's no way
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to delegate one.
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- **Asynchronous / buffered send** for the cases where a rendezvous is the wrong
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shape (logging, notifications between servers). *Landed as `ipc_send`* — a
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non-blocking post to an endpoint's bounded payload queue, delivered through
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`reply_wait` as a buffered message (badge bit `notify_message_bit`). Built for, and
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first used by, the [input service](input.md)'s keyboard-event broadcast, where a
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synchronous push would let one dead subscriber hang the fan-out. A full queue drops
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the oldest (discrete messages, not a coalescing level like the notification ring).
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- **A bounded reply.** `MSG_MAX` is 256 bytes and the copy runs under the big kernel
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lock; a bulk transfer wants shared pages, not a copy.
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## Lifecycle conventions over IPC (M17)
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Three conventions from [process-lifecycle.md](process-lifecycle.md) ride the
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notification mechanism:
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- **Signals** arrive as notifications on the endpoint a process nominated with
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`signal_bind` (`runtime.process.bindSignals`): badge = the signal bit plus the
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coalesced pending mask (`runtime.process.signalsFrom` decodes). Statements,
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never questions; no payload, no reply.
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- **One-shot timers** (`timer_bind`, `runtime.system.timerOnce`) land as a
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timer-bit notification — the timed wait: a service arms a deadline and keeps
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serving, instead of blocking in sleep.
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- **The universal ping**: a **zero-length request is the liveness probe**,
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answered with a zero-length reply by the service harness itself
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(`runtime.service.run`). No protocol's requests start at length zero, so the
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encoding cannot collide, and a wedged service simply fails to answer — which
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is the diagnosis. Deep health ("can I reach my hardware?") stays a per-service
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protocol message.
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