uACPI

A portable and easy-to-integrate implementation of the Advanced Configuration and Power Interface (ACPI).

Features

• A fast and well-tested AML interpreter optimized to use very little stack space
• NT-compatible on a fundamental level (see examples)
• Very easy to integrate (ships with own overridable standard library implementation)
• Highly flexible and configurable (optional sized frees, reduced-hw-only mode, etc.)
• A fairly advanced event subsystem (GPE/fixed, wake, implicit notify, AML handlers)
• Table management API (search, dynamic installation/loading, overrides, etc.)
• Operation region subsystem (user handlers, support for BufferAcc opregions, builtins for common types)
• Sleep state management (transition to any S state, wake vector programming)
• PCI routing table retrieval & interrupt model API
• Device search API
• Resource subsystem supporting every resource defined by ACPI 6.5
• Interface & feature management exposed via _OSI
• Client-defined Notify() handlers
• Firmware global lock management (_GL, locked fields, public API)
• GAS read/write API
• Fully thread safe
• Supports both 32-bit and 64-bit platforms
• A special barebones mode with only table API (see config.h)

Why would I use this over ACPICA?

1. NT-compatible from the ground up

Over the decades of development, ACPICA has accumulated a lot of workarounds for
AML expecting NT-specific behaviors, and is still missing compatibility in a lot
of critical aspects.

uACPI, on the other hand, is built to be natively NT-compatible without extra
workarounds.

Some specific highlights include:

• Reference objects, especially multi-level reference chains
• Implicit cast semantics
• Object mutability
• Named object resolution, especially for named objects inside packages

2. Fundamental safety

uACPI is built to always assume the worst about the AML byte code it’s executing,
and as such, has a more sophisticated object lifetime tracking system, as well
as carefully designed handling for various edge-cases, including race conditions.

Some of the standard uACPI test cases crash both ACPICA, and the NT AML
interpreters.

While a permanent fuzzing solution for uACPI is currently WIP, it has already
been fuzzed quite extensively and all known issues have been fixed.

3. No recursion

Running at kernel level has a lot of very strict limitations, one of which is a
tiny stack size, which can sometimes be only a few pages in length.

Of course, both ACPICA and uACPI have non-recursive AML interpreters, but there
are still edge cases that cause potentially unbounded recursion.

One such example are the dynamic table load operators from AML
(Load/LoadTable): these cause a linear growth in stack usage per call in
ACPICA, whereas in uACPI these are treated as special method calls,
and as such, don’t increase stack usage whatsoever.

More detailed overview

Expressions within package:

Method (TEST) {
    Local0 = 10
    Local1 = Package { Local0 * 5 }
    Return (DerefOf(Local1[0]))
}

// ACPICA: AE_SUPPORT, Expressions within package elements are not supported
// Windows, uACPI: Local0 = 50
Local0 = TEST()

Packages outside of a control method:

// ACPICA: internal error
// Windows, uACPI: ok
Local0 = Package { 1 }

Reference rebind semantics:

Local0 = 123
Local1 = RefOf(Local0)

// ACPICA: Local1 = 321, Local0 = 123
// Windows, uACPI: Local1 = reference->Local0, Local0 = 321
Local1 = 321

Increment/Decrement:

Local0 = 123
Local1 = RefOf(Local0)

// ACPICA: error
// Windows, uACPI: Local0 = 124
Local1++

Multilevel references:

Local0 = 123
Local1 = RefOf(Local0)
Local2 = RefOf(Local1)

// ACPICA: Local3 = reference->Local0
// Windows, uACPI: Local3 = 123
Local3 = DerefOf(Local2)

Implict-cast semantics:

Name (TEST, "BAR")

// ACPICA: TEST = "00000000004F4F46"
// Windows, uACPI: TEST = "FOO"
TEST = 0x4F4F46

Buffer size mutability:

Name (TEST, "XXXX")
Name (VAL, "")

// ACPICA: TEST = "LONGSTRING"
// Windows, UACPI: TEST = "LONG"
TEST = "LONGSTRING"

// ACPICA: VAL = "FOO"
// Windows, UACPI: VAL = ""
VAL = "FOO"

Returning a reference to a local object:

Method (TEST) {
    Local0 = 123

    // Use-after-free in ACPICA, perfectly fine in uACPI
    Return (RefOf(Local0))
}

Method (FOO) {
    Name (TEST, 123)

    // Use-after-free in ACPICA, object lifetime prolonged in uACPI (node is still removed from the namespace)
    Return (RefOf(TEST))
}

CopyObject into self:

Method (TEST) {
    CopyObject(123, TEST)
    Return (1)
}

// Segfault in ACPICA, prints 1 in uACPI  
Debug = TEST()

// Unreachable in ACPICA, prints 123 in uACPI
Debug = TEST

There’s even more examples, but this should be enough to demonstrate the fundamental differences in designs.

Integrating into a kernel

1. Add uACPI sources & include directories into your project

If you’re using CMake

Simply add the following lines to your cmake:

include(uacpi/uacpi.cmake)

target_sources(
    my-kernel
    PRIVATE
    ${UACPI_SOURCES}
)

target_include_directories(
    my-kernel
    PRIVATE
    ${UACPI_INCLUDES}
)

If you’re using Meson

Add the following lines to your meson.build:

uacpi = subproject('uacpi')

uacpi_sources = uacpi.get_variable('sources')
my_kernel_sources += uacpi_sources

uacpi_includes = uacpi.get_variable('includes')
my_kernel_includes += uacpi_includes

Any other build system

• Add all .c files from source into your target sources
• Add include into your target include directories

2. Implement/override platform-specific headers

uACPI defines all platform/architecture-specific functionality in a few headers inside include/uacpi/platform

All of the headers can be “implemented” by your project in a few ways:

• Implement the expected helpers exposed by the headers
• Replace the expected helpers by your own and override uACPI to use them by defining the respective UACPI_OVERRIDE_X variable.
  In this case, the header becomes a proxy that includes a corresponding uacpi_x.h header exported by your project.

Currently used platform-specific headers are:

• arch_helpers.h - defines architecture/cpu-specific helpers & thread-id-related interfaces
• compiler.h - defines compiler-specific helpers like attributes and intrinsics.
  This already works for MSVC, clang & GCC so you most likely won’t have to override it.
• atomic.h - defines compiler-specific helpers for dealing with atomic operations.
  Same as the header above, this should work out of the box for MSVC, clang & GCC.
• libc.h - an empty header by default, but may be overriden by your project
  if it implements any of the libc functions used by uACPI (by default uACPI uses its
  own implementations to be platform-independent and to make porting easier). The
  internal implementation is just the bare minimum and not optimized in any way.
• types.h - typedefs a bunch of uacpi-specific types using the stdint.h header. You don’t have to override this
  unless you don’t provide stdint.h.
• config.h - various compile-time options and settings, preconfigured to reasonable defaults.

3. Implement kernel API

uACPI relies on kernel-specific API to do things like mapping/unmapping memory, writing/reading to/from IO, PCI config space, and many more things.

This API is declared in kernel_api.h and is implemented by your kernel.

4. Initialize uACPI

That’s it, uACPI is now integrated into your project.

You should proceed to initialization.
Refer to the uACPI page on osdev wiki to see a
snippet for basic initialization, as well as some code examples of how you may
want to use certain APIs.

All of the headers and APIs defined in uacpi are public and may be utilized by your project.
Anything inside uacpi/internal is considered private/undocumented and unstable API.

Developing and contributing

Most development work is fully doable in userland using the test runner.

Setting up an IDE:

Simply open tests/runner/CMakeLists.txt in your favorite IDE.

For Visual Studio:

cd tests\runner && mkdir build && cd build && cmake ..

Then just simply open the .sln file generated by cmake.

Running the test suite:

./tests/run_tests.py

If you want to contribute:

• Commits are expected to be atomic (changing one specific thing, or introducing one feature) with detailed description (if one is warranted for), an S-o-b line is welcome
• Code style is 4-space tabs, 80 cols, the rest can be seen by just looking at the current code

All contributions are very welcome!

Notable projects using uACPI & performance leaderboards

Project	Description	(qemu w/ Q35 + KVM) ops/s	CPU
proxima	A monolithic Unix-like operating system	8,468,599	Intel Core i9-13900KS
Crescent2	An NT driver compatible kernel and userspace	6,818,418	Intel Core i5-13600K
davix	Yet another unix-like by some bored nerd	6,364,623	Intel Core i7-13700K
ilobilix	Yet another monolithic Linux clone wannabe. Currently under a rewrite	5,960,392	Intel Core i9-13900KS
Managarm	Pragmatic microkernel-based OS with fully asynchronous I/O	5,618,646	Intel Core i7-14700K
pmOS	Microkernel-based operating system written from scratch with uACPI running in userspace	5,354,445	AMD Ryzen 9 5900X
menix	A minimal and expandable Unix-like operating system	5,239,043	Intel Core Ultra 7 265KF
Ironclad	Formally verified, hard real-time capable kernel written in SPARK and Ada	4,802,816	Intel Core i9-13900KS
Astral	Operating system written in C which aims be POSIX-compliant	4,189,189	Intel Core i5-13600K
Keyronex	Layered kernel with fundamentally asynchronous I/O and working set model-based memory management	4,013,691	AMD Ryzen 5800X
Orange	x86_64 Unix-like OS	2,377,330	AMD Ryzen 5 3600
OBOS	Hybrid Kernel with advanced driver loading	2,141,179	Intel Core i5-13600K
ElysiumOS	Hybrid Unix-like kernel	1,737,654	AMD Ryzen 7 5800X3D
imaginarium	Ziggy osdev experiments inspired by the NT kernel (using the zig general purpose allocator)	1,504,436	AMD Ryzen 7 3700X
BadgerOS	A monolithic lightweight UNIX clone	1,018,518	AMD Ryzen 5 3600
NyauxKC	Monolithic UNIX-like multi-architecture kernel	985,988	Intel Core Ultra 7 265K

License

uACPI is licensed under the MIT License.
The full license text is provided in the LICENSE file inside the root directory.