kernel_messages.txt
Barret Rhoden
2010-03-19
Updated 2012-11-14

This document explains the basic ideas behind our "kernel messages" (KMSGs) and
some of the arcane bits behind the implementation.  These were formerly called
active messages, since they were an implementation of the low-level hardware
messaging.

Overview:
--------------------------------
Our kernel messages are just work that is shipped remotely, delayed in time, or
both.  They currently consist of a function pointer and a few arguments.  Kernel
messages of a given type will be executed in order, with guaranteed delivery.

Initially, they were meant to be a way to immediately execute code on another
core (once interrupts are enabled), in the order in which the messages were
sent.  This is insufficient (and wasn't what we wanted for the task,
incidentally).  We simply want to do work on another core, but not necessarily
instantly.  And not necessarily on another core.

Currently, there are two types, distinguished by which list they are sent to per
core: immediate and routine.   Routine messages are often referred to as RKMs.
Immediate messages will get executed as soon as possible (once interrupts are
enabled).  Routine messages will be executed at convenient points in the kernel.
This includes when the kernel is about to pop back to userspace
(proc_restartcore()), or smp_idle()ing.  Routine messages are necessary when
their function does not return, such as a __launch_kthread.  They should also be
used if the work is not worth fully interrupting the kernel.  (An IPI will still
be sent, but the work will be delayed).  Finally, they should be used if their
work could affect currently executing kernel code (like a syscall).

For example, some older KMSGs such as __startcore used to not return and would
pop directly into user space.  This complicted the KMSG code quite a bit.  While
these functions now return, they still can't be immediate messages.  Proc
management KMSGs change the cur_ctx out from under a syscall, which can lead to
a bunch of issues.

Immediate kernel messages are executed in interrupt context, with interrupts
disabled.  Routine messages are only executed from places in the code where the
kernel doesn't care if the functions don't return or otherwise cause trouble.
This means RKMs aren't run in interrupt context in the kernel (or if the kernel
code itself traps).  We don't have a 'process context' like Linux does, instead
its more of a 'default context'.  That's where RKMs run, and they run with IRQs
disabled.

RKMs can enable IRQs, or otherwise cause IRQs to be enabled.  __launch_kthread
is a good example: it runs a kthread, which may have had IRQs enabled.

With RKMs, there are no concerns about the kernel holding locks or otherwise
"interrupting" its own execution.  Routine messages are a little different than
just trapping into the kernel, since the functions don't have to return and may
result in clobbering the kernel stack.  Also note that this behavior is
dependent on where we call process_routine_kmsg().  Don't call it somewhere you
need to return to.

An example of an immediate message would be a TLB_shootdown.  Check current,
flush if applicable, and return.  It doesn't harm the kernel at all.  Another
example would be certain debug routines.

History:
--------------------------------
KMSGs have a long history tied to process management code.  The main issues were
related to which KMSG functions return and which ones mess with local state (like
clobbering cur_ctx or the owning_proc).  Returning was a big deal because you
can't just arbitrarily abandon a kernel context (locks or refcnts could be held,
etc).  This is why immediates must return.  Likewise, there are certain
invariants about what a core is doing that shouldn't be changed by an IRQ
handler (which is what an immed message really is).  See all the old proc
management commits if you want more info (check for changes to __startcore).

Other Uses:
--------------------------------
Kernel messages will also be the basis for the alarm system.  All it is is
expressing work that needs to be done.  That being said, the k_msg struct will
probably receive a timestamp field, among other things.  Routine messages also
will replace the old workqueue, which hasn't really been used in 40 months or
so.

To Return or Not:
--------------------------------
Routine k_msgs do not have to return.  Immediate messages must.  The distinction
is in how they are sent (send_kernel_message() will take a flag), so be careful.

To retain some sort of sanity, the functions that do not return must adhere to
some rules.  At some point they need to end in a place where they check routine
messages or enable interrupts.  Simply calling smp_idle() will do this.  The
idea behind this is that routine messages will get processed once the kernel is
able to (at a convenient place). 

Missing Routine Messages:
--------------------------------
It's important that the kernel always checks for routine messages before leaving
the kernel, either to halt the core or to pop into userspace.  There is a race
involved with messages getting posted after we check the list, but before we
pop/halt.  In that time, we send an IPI.  This IPI will force us back into the
kernel at some point in the code before process_routine_kmsg(), thus keeping us
from missing the RKM.

In the future, if we know the kernel code on a particular core is not attempting
to halt/pop, then we could avoid sending this IPI.  This is the essence of the
optimization in send_kernel_message() where we don't IPI ourselves.  A more
formal/thorough way to do this would be useful, both to avoid bugs and to
improve cross-core KMSG performance.

IRQ Trickiness:
--------------------------------
You cannot enable interrupts in the handle_kmsg_ipi() handler, either in the
code or in any immediate kmsg.  Since we send the EOI before running the handler
(on x86), another IPI could cause us to reenter the handler, which would spin on
the lock the previous context is holding (nested IRQ stacks).  Using irqsave
locks is not sufficient, since they assume IRQs are not turned on in the middle
of their operation (such as in the body of an immediate kmsg).

Other Notes:
--------------------------------
Unproven hunch, but the main performance bottleneck with multiple senders and
receivers of k_msgs will be the slab allocator.  We use the slab so we can
dynamically create the k_msgs (can pass them around easily, delay with them
easily (alarms), and most importantly we can't deadlock by running out of room
in a static buffer).

Architecture Dependence:
--------------------------------
Some details will differ, based on architectural support.  For instance,
immediate messages can be implemented with true active messages.  Other systems
with maskable IPI vectors can use a different IPI for routine messages, and that
interrupt can get masked whenever we enter the kernel (note, that means making
every trap gate an interrupt gate), and we unmask that interrupt when we want to
process routine messages.

However, given the main part of kmsgs is arch-independent, I've consolidated all
of it in one location until we need to have separate parts of the implementation.