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intro(9): rewrite from scratch

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This page has existed as a placeholder since its creation in 1995. It
does not provide a useful introduction to the content in this section.

Reimagine it as a top-level overview page containing brief descriptions
and links to existing pages in section 9. It is roughly organized into
sub-sections, grouped by topic or subsystem. In other words, the page is
meant to function as a map to other content.

There is a balance to be found here between providing as many links as
possible and keeping the page concise and searchable. In general the aim
is to reference pages which provide the best entry point to a particular
topic. For example, a link is given to locking(9), but not to the
specific lock pages such as mutex(9) or rwlock(9).

NetBSD has done something similar with their intro(9), so some
inspiration has been taken from there, although their content doesn't
align that closely with what we have.

I have done a thorough review of our existing pages and formed these
subsections around them, but they are meant to evolve.

PR:		270481
Reviewed by:	imp, emaste
MFC after:	3 weeks
Relnotes:	yes
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D41104
This commit is contained in:
Mitchell Horne 2023-08-03 10:48:15 -03:00
parent d441ec1c21
commit 84f9f2c5cf
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.\" Copyright (c) 1983, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\" Copyright (c) 2023 The FreeBSD Foundation
.\"
.\" $FreeBSD$
.\" This manual page was written by Mitchell Horne <mhorne@FreeBSD.org> under
.\" sponsorship from the FreeBSD Foundation.
.\"
.Dd December 13, 1995
.Dd August 2, 2023
.Dt INTRO 9
.Os
.Sh NAME
.Nm intro
.Nd "introduction to system kernel interfaces"
.Nd "introduction to kernel programming interfaces"
.Sh DESCRIPTION
This section contains information about the interfaces and
subroutines in the kernel.
.Sh PROTOTYPES ANSI-C AND ALL THAT
Yes please.
Welcome to the
.Fx
kernel documentation.
Outside of the source code itself, this set of
.Xr man 1
pages is the primary resource for information on usage of the numerous
programming interfaces available within the kernel.
In some cases, it is also a source of truth for the implementation details
and/or design decisions behind a particular subsystem or piece of code.
.Pp
We would like all code to be fully prototyped.
The intended audience of this documentation is developers, and the primary
authors are also developers.
It is written assuming a certain familiarity with common programming or
OS-level concepts and practices.
However, this documentation should also attempt to provide enough background
information that readers approaching a particular subsystem or interface for
the first time will be able to understand.
.Pp
If your code compiles cleanly with
.Nm cc
.Ar -Wall
we would feel happy about it.
It is important to understand that this is not a question of just shutting up
.Nm cc ,
it is a question about avoiding the things it complains about.
To put it bluntly, do not hide the problem by casting and other
obfuscating practices, solve the problem.
.Sh INDENTATION AND STYLE
Believe it or not, there actually exists a guide for indentation and style.
It is not generally applied though.
To further set expectations, we acknowledge that kernel documentation, like the
source code itself, is forever a work-in-progress.
There will be large sections of the codebase whose documentation is subtly or
severely outdated, or missing altogether.
This documentation is a supplement to the source code, and can not always be
taken at face value.
.Pp
We would appreciate if people would pay attention to it, and at least not
violate it blatantly.
At its best, section 9 documentation will provide a description of a particular
piece of code that, paired with its implementation, fully informs the reader of
the intended and realized effects.
.Pp
We do not mind it too badly if you have your own style, but please make
sure we can read it too.
.Pp
Please take time to read
.Xr man 1
pages in this section most frequently describe functions, but may also
describe types, global variables, macros, or high-level concepts.
.Sh CODING GUIDELINES
Code written for the
.Fx
kernel is expected to conform to the established style and coding conventions.
Please see
.Xr style 9
for more information.
.Sh NAMING THINGS
Some general rules exist:
.Bl -enum
.It
If a function is meant as a debugging aid in DDB, it should be enclosed
in
.Bd -literal -offset indent
#ifdef DDB
#endif /* DDB */
for a detailed set of rules and guidelines.
.Sh OVERVIEW
Below is presented various subsystems.
.Ss Data Structures
There are implementations for many well-known data structures available in the
kernel.
.Bl -tag -width "Xr bitstring 3"
.It Xr bitstring 3
Simple bitmap implementation.
.It Xr counter 9
An SMP-safe general-purpose counter implementation.
.It Xr hash 9
Hash map implementation.
.It Xr nv 9
Name/value pairs.
.It Xr queue 3
Singly-linked and doubly-linked lists, and queues.
.It Xr refcount 9
An SMP-safe implementation of reference counts.
.It Xr sbuf 9
Dynamic string composition.
.It Xr sglist 9
A scatter/gather list implementation.
.El
.Ss Utility Functions
Functions or facilities of general usefulness or convenience.
See also the
.Sx Testing and Debugging Tools
or
.Sx Miscellaneous
sub-sections below.
.Pp
Formatted output and logging functions are described by
.Xr printf 9 .
.Pp
Endian-swapping functions:
.Xr byteorder 9 .
.Pp
Data output in hexadecimal format:
.Xr hexdump 9 .
.Pp
A rich set of macros for declaring
.Xr sysctl 8
variables and functions is described by
.Xr sysctl 9 .
.Pp
Non-recoverable errors in the kernel should trigger a
.Xr panic 9 .
Run-time assertions can be verified using the
.Xr KASSERT 9
macros.
Compile-time assertions should use
.Fn _Static_assert .
.Pp
The SYSINIT framework provides macros for declaring functions that will be
executed during start-up and shutdown; see
.Xr SYSINIT 9 .
.Pp
Deprecation messages may be emitted with
.Xr gone_in 9 .
.Pp
A unit number facility is provided by
.Xr unr 9 .
.Ss Synchronization Primitives
The
.Xr locking 9
man page gives an overview of the various types of locks available in the
kernel and advice on their usage.
.Pp
Atomic primitives are described by
.Xr atomic 9 .
.Pp
The
.Xr epoch 9
and
.Xr smr 9
facilities are used to create lock-free data structures.
There is also
.Xr seqc 9 .
.Ss Memory Management
Dynamic memory allocations inside the kernel are generally done using
.Xr malloc 9 .
Frequently allocated objects may prefer to use
.Xr uma 9 .
.Pp
.\" MHTODO: It would be useful to have a vm_page(9) or similar
.\" high-level page which points to the following contents instead.
Much of the virtual memory system operates on
.Vt vm_page_t
structures.
The following functions are documented:
.Bd -ragged -offset indent
.Xr vm_page_advise 9 ,
.Xr vm_page_alloc 9 ,
.Xr vm_page_bits 9 ,
.Xr vm_page_aflag 9 ,
.Xr vm_page_alloc 9 ,
.Xr vm_page_bits 9 ,
.Xr vm_page_busy 9 ,
.Xr vm_page_deactivate 9 ,
.Xr vm_page_free 9 ,
.Xr vm_page_grab 9 ,
.Xr vm_page_insert 9 ,
.Xr vm_page_lookup 9 ,
.Xr vm_page_rename 9 ,
.Xr vm_page_sbusy 9 ,
.Xr vm_page_wire 9
.Ed
.Pp
And the name of the procedure should start with the prefix
.Li DDB_
to clearly identify the procedure as a debugger routine.
.El
.Sh SCOPE OF SYMBOLS
It is important to carefully consider the scope of symbols in the kernel.
The default is to make everything static, unless some reason requires
the opposite.
Virtual address space maps are managed with the
.Xr vm_map 9
API.
.Pp
There are several reasons for this policy,
the main one is that the kernel is one monolithic name-space,
and pollution is not a good idea here either.
The machine-dependent portion of the virtual memory stack is the
.Xr pmap 9
module.
.Pp
For device drivers and other modules that do not add new internal interfaces
to the kernel, the entire source should be in one file if possible.
That way all symbols can be made static.
Allocation policies for NUMA memory domains are managed with the
.Xr domainset 9
API.
.Ss File Systems
The kernel interface for file systems is
.Xr VFS 9 .
File system implementations register themselves with
.Xr vfsconf 9 .
.Pp
The abstract and filesystem-independent representation of a file, directory, or
other file-like entity within the kernel is the
.Xr vnode 9 .
.Pp
The implementation of access control lists for filesystems is described by
.Xr acl 9 .
Also
.Xr vaccess 9 .
.Ss I/O and Storage
.\" TODO: This page needs to be rewritten before it can be included here.
.\" The buffer cache is described by:
.\" .Xr buf 9
.\" .Pp
The GEOM framework represents I/O requests using the
.Xr bio 9
structure.
.Pp
Disk drivers connect themselves to GEOM using the
.Xr disk 9
API.
.Pp
If for some reason a module is split over multiple source files, then try
to split the module along some major fault-line and consider using the
number of global symbols as your guide.
The fewer the better.
.Sh SEE ALSO
.Xr style 9
.Sh HISTORY
The
.Nm
section manual page appeared in
.Fx 2.2 .
.Xr devstat 9
facility provides an interface for recording device statistics in disk drivers.
.Ss Networking
Much of the networking stack uses the
.Xr mbuf 9 ,
a flexible memory management unit commonly used to store network packets.
.Pp
Network interfaces are implemented using the
.Xr ifnet 9
API, which has functions for drivers and consumers.
.Pp
A framework for managing packet output queues is described by
.Xr altq 9 .
.Pp
To receive incoming packets, network protocols register themselves with
.Xr netisr 9 .
.Pp
Virtualization of the network stack is provided by
.Xr VNET 9 .
.Pp
The front-end for interfacing with network sockets from within the kernel is
described by
.Xr socket 9 .
The back-end interface for socket implementations is
.Xr domain 9 .
.Pp
The low-level packet filter interface is described by
.Xr pfil 9 .
.Pp
The
.Xr bpf 9
interface provides a mechanism to redirect packets to userspace.
.Pp
The subsystem for IEEE 802.11 wireless networking is described by
.Xr ieee80211 9 .
.Pp
A framework for modular TCP implementations is described by
.Xr tcp_functions 9 .
.Pp
A framework for modular congestion control algorithms is described by
.Xr mod_cc 9 .
.Ss Device Drivers
.\" TODO: a bus(9) or newbus(9) page, as well as updates to existing pages
.\" would be helpful in laying out the high-level concepts of FreeBSD's device
.\" structure, and explaining the organization of existing documentation.
Consult the
.Xr device 9
and
.Xr driver 9
pages first.
.Pp
Most drivers act as devices, and provide a set of methods implementing the
device interface.
This includes methods such as
.Xr DEVICE_PROBE 9 ,
.Xr DEVICE_ATTACH 9 ,
and
.Xr DEVICE_DETACH 9 .
.Pp
In addition to devices, there are buses.
Buses may have children, in the form of devices or other buses.
Bus drivers will implement additional methods, such as
.Xr BUS_ADD_CHILD 9 ,
.Xr BUS_READ_IVAR 9 ,
or
.Xr BUS_RESCAN 9 .
.Pp
Buses often perform resource accounting on behalf of their children.
For this there is the
.Xr rman 9
API.
.Pp
Drivers can request and manage their resources (e.g. memory-space or IRQ
number) from their parent using the following sets of functions:
.Bd -ragged -offset indent
.Xr bus_alloc_resource 9 ,
.Xr bus_adjust_resource 9 ,
.Xr bus_get_resource 9 ,
.Xr bus_map_resource 9 ,
.Xr bus_release_resource 9 ,
.Xr bus_set_resource 9
.Ed
.Pp
Direct Memory Access (DMA) is handled using the
.Xr busdma 9
framework.
.Pp
Functions for accessing bus space (i.e. read/write) are provided by
.Xr bus_space 9 .
.Ss Clocks and Timekeeping
The kernel clock frequency and overall system time model is described by
.Xr hz 9 .
.Pp
A few global time variables, such as system up-time, are described by
.Xr time 9 .
.Pp
Raw CPU cycles are provided by
.Xr get_cyclecount 9 .
.Ss Userspace Memory Access
Direct read/write access of userspace memory from the kernel is not permitted,
and memory transactions that cross the kernel/user boundary must go through one
of several interfaces built for this task.
.Pp
Most device drivers use the
.Xr uiomove 9
set of routines.
.Pp
Simpler primitives for reading or writing smaller chunks of memory are
described by
.Xr casuword 9 ,
.Xr copy 9 ,
.Xr fetch 9 ,
and
.Xr store 9 .
.Ss Kernel Threads, Tasks, and Callbacks
Kernel threads and processes are created using the
.Xr kthread 9
and
.Xr kproc 9
interfaces, respectively.
.Pp
Where dedicated kernel threads are too heavyweight, there is also the
.Xr taskqueue 9
interface.
.Pp
For low-latency callback handling, the
.Xr callout 9
framework should be used.
.Pp
Dynamic handlers for pre-defined event hooks are registered and invoked using
the
.Xr EVENTHANDLER 9
API.
.Ss Thread Switching and Scheduling
The machine-independent interface to a context switch is
.Xr mi_switch 9 .
.Pp
To prevent preemption, use a
.Xr critical 9
section.
.Pp
To voluntarily yield the processor, use
.Xr kern_yield 9 .
.Pp
The various functions which will deliberately put a thread to sleep are
described by
.Xr sleep 9 .
Sleeping threads are removed from the scheduler and placed on a
.Xr sleepqueue 9 .
.\" TODO: This page is outdated and can't be included here yet.
.\".Pp
.\"The thread scheduler interface is described by
.\".Xr scheduler 9 .
.Ss Processes and Signals
To locate a process or process group by its identifier, use
.Xr pfind 9
and
.Xr pgfind 9 .
Alternatively, the
.Xr pget 9
function provides additional search specificity.
.Pp
The "hold count" of a process can be manipulated with
.Xr PHOLD 9 .
.Pp
The kernel interface for signals is described by
.Xr signal 9 .
.Pp
Signals can be sent to processes or process groups using the functions
described by
.Xr psignal 9 .
.Ss Security
See the generic security overview in
.Xr security 7 .
.Pp
The basic structure for user credentials is
.Vt struct ucred ,
managed by the
.Xr ucred 9
API.
Thread credentials are verified using
.Xr priv 9
to allow or deny certain privileged actions.
.Pp
Policies influenced by
.Va kern.securelevel
must use the
.Xr securelevel_gt 9
or
.Xr securelevel_ge 9
functions.
.Pp
The Mandatory Access Control (MAC) framework provides a wide set of hooks,
supporting dynamically-registered security modules;
see
.Xr mac 9 .
.Pp
Cryptographic services are provided by the OpenCrypto framework.
This API provides and interface for both consumers and crypto drivers;
see
.Xr crypto 9 .
.Pp
For information on random number generation, see
.Xr random 9
and
.Xr prng 9 .
.Ss Kernel Modules
The interfaces for declaring loadable kernel modules are described by
.Xr module 9 .
.Ss Interrupts
The machine-independent portion of the interrupt framework supporting the
registration and execution of interrupt handlers is described by
.Xr intr_event 9 .
.Pp
Software interrupts are provided by
.Xr swi 9 .
.Pp
Device drivers register their interrupt handlers using the
.Xr bus_setup_intr 9
function.
.Ss Testing and Debugging Tools
A kernel test framework:
.Xr kern_testfrwk 9
.Pp
A facility for defining configurable fail points is described by
.Xr fail 9 .
.Pp
Commands for the
.Xr ddb 4
kernel debugger are defined with the
.Xr DB_COMMAND 9
family of macros.
.Pp
The
.Xr ktr 4
tracing facility adds static tracepoints to many areas of the kernel.
These tracepoints are defined using the macros described by
.Xr ktr 9 .
.Pp
Static probes for DTrace are defined using the
.Xr SDT 9
macros.
.Pp
Stack traces can be captured and printed with the
.Xr stack 9
API.
.Pp
Kernel sanitizers can perform additional compiler-assisted checks against
memory use/access.
These runtimes are capable of detecting difficult-to-identify classes of bugs,
at the cost of a large overhead.
Supported is the Kernel Address Sanitizer
.Xr KASAN 9 ,
and the Kernel Memory Sanitizer
.Xr KMSAN 9 .
.Pp
The
.Cd LOCK_PROFILING
kernel config option enables extra code to assist with profiling and/or
debugging lock performance;
see
.Xr LOCK_PROFILING 9 .
.Ss Driver Tools
Defined functions/APIs for specific types of devices.
.Bl -tag -width "Xr usbdi 9"
.It Xr iflib 9
Programming interface for
.Xr iflib 4
based network drivers.
.It Xr pci 9
Peripheral Component Interconnect (PCI) and PCI Express (PCIe) programming API.
.It Xr pwmbus 9
Pulse-Width Modulation (PWM) bus interface methods.
.It Xr usbdi 9
Universal Serial Bus programming interface.
.It Xr superio 9
Functions for Super I/O controller devices.
.El
.Ss Miscellaneous
Dynamic per-CPU variables:
.Xr dpcpu 9 .
.Pp
CPU bitmap management:
.Xr cpuset 9 .
.Pp
Kernel environment management:
.Xr getenv 9 .
.Pp
Contexts for CPU floating-point registers are managed by the
.Xr fpu_kern 9
facility.
.Pp
For details on the shutdown/reboot procedure and available shutdown hooks, see
.Xr reboot 9 .
.Pp
A facility for asynchronous logging to files from within the kernel is provided
by
.Xr alq 9 .
.Pp
The
.Xr osd 9
framework provides a mechanism to dynamically extend core structures in a way
that preserves KBI.
See the
.Xr hhook 9
and
.Xr khelp 9
APIs for information on how this is used.
.Pp
The kernel object implementation is described by
.Xr kobj 9 .
.Sh SEE ALSO
.Xr man 1 ,
.Xr style 9
.Rs
.%T "The FreeBSD Architecture Handbook"
.%U "https://docs.freebsd.org/en/books/arch-handbook/"
.Re