From 6cbd1d6d36c5d8312de99d1dfa3bec40ac840ce0 Mon Sep 17 00:00:00 2001 From: Samuel Holland Date: Fri, 29 Mar 2024 00:18:16 -0700 Subject: arch: add ARCH_HAS_KERNEL_FPU_SUPPORT MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Several architectures provide an API to enable the FPU and run floating-point SIMD code in kernel space. However, the function names, header locations, and semantics are inconsistent across architectures, and FPU support may be gated behind other Kconfig options. provide a standard way for architectures to declare that kernel space FPU support is available. Architectures selecting this option must implement what is currently the most common API (kernel_fpu_begin() and kernel_fpu_end(), plus a new function kernel_fpu_available()) and provide the appropriate CFLAGS for compiling floating-point C code. Link: https://lkml.kernel.org/r/20240329072441.591471-2-samuel.holland@sifive.com Signed-off-by: Samuel Holland Suggested-by: Christoph Hellwig Reviewed-by: Christoph Hellwig Acked-by: Christian König Cc: Alex Deucher Cc: Borislav Petkov (AMD) Cc: Catalin Marinas Cc: Dave Hansen Cc: Huacai Chen Cc: Ingo Molnar Cc: Jonathan Corbet Cc: Masahiro Yamada Cc: Michael Ellerman Cc: Nathan Chancellor Cc: Nicolas Schier Cc: Palmer Dabbelt Cc: Russell King Cc: Thomas Gleixner Cc: WANG Xuerui Cc: Will Deacon Signed-off-by: Andrew Morton --- Documentation/core-api/floating-point.rst | 78 +++++++++++++++++++++++++++++++ Documentation/core-api/index.rst | 1 + 2 files changed, 79 insertions(+) create mode 100644 Documentation/core-api/floating-point.rst (limited to 'Documentation/core-api') diff --git a/Documentation/core-api/floating-point.rst b/Documentation/core-api/floating-point.rst new file mode 100644 index 000000000000..a8d0d4b05052 --- /dev/null +++ b/Documentation/core-api/floating-point.rst @@ -0,0 +1,78 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +Floating-point API +================== + +Kernel code is normally prohibited from using floating-point (FP) registers or +instructions, including the C float and double data types. This rule reduces +system call overhead, because the kernel does not need to save and restore the +userspace floating-point register state. + +However, occasionally drivers or library functions may need to include FP code. +This is supported by isolating the functions containing FP code to a separate +translation unit (a separate source file), and saving/restoring the FP register +state around calls to those functions. This creates "critical sections" of +floating-point usage. + +The reason for this isolation is to prevent the compiler from generating code +touching the FP registers outside these critical sections. Compilers sometimes +use FP registers to optimize inlined ``memcpy`` or variable assignment, as +floating-point registers may be wider than general-purpose registers. + +Usability of floating-point code within the kernel is architecture-specific. +Additionally, because a single kernel may be configured to support platforms +both with and without a floating-point unit, FPU availability must be checked +both at build time and at run time. + +Several architectures implement the generic kernel floating-point API from +``linux/fpu.h``, as described below. Some other architectures implement their +own unique APIs, which are documented separately. + +Build-time API +-------------- + +Floating-point code may be built if the option ``ARCH_HAS_KERNEL_FPU_SUPPORT`` +is enabled. For C code, such code must be placed in a separate file, and that +file must have its compilation flags adjusted using the following pattern:: + + CFLAGS_foo.o += $(CC_FLAGS_FPU) + CFLAGS_REMOVE_foo.o += $(CC_FLAGS_NO_FPU) + +Architectures are expected to define one or both of these variables in their +top-level Makefile as needed. For example:: + + CC_FLAGS_FPU := -mhard-float + +or:: + + CC_FLAGS_NO_FPU := -msoft-float + +Normal kernel code is assumed to use the equivalent of ``CC_FLAGS_NO_FPU``. + +Runtime API +----------- + +The runtime API is provided in ``linux/fpu.h``. This header cannot be included +from files implementing FP code (those with their compilation flags adjusted as +above). Instead, it must be included when defining the FP critical sections. + +.. c:function:: bool kernel_fpu_available( void ) + + This function reports if floating-point code can be used on this CPU or + platform. The value returned by this function is not expected to change + at runtime, so it only needs to be called once, not before every + critical section. + +.. c:function:: void kernel_fpu_begin( void ) + void kernel_fpu_end( void ) + + These functions create a floating-point critical section. It is only + valid to call ``kernel_fpu_begin()`` after a previous call to + ``kernel_fpu_available()`` returned ``true``. These functions are only + guaranteed to be callable from (preemptible or non-preemptible) process + context. + + Preemption may be disabled inside critical sections, so their size + should be minimized. They are *not* required to be reentrant. If the + caller expects to nest critical sections, it must implement its own + reference counting. diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index 7a3a08d81f11..974beccd671f 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -48,6 +48,7 @@ Library functionality that is used throughout the kernel. errseq wrappers/atomic_t wrappers/atomic_bitops + floating-point Low level entry and exit ======================== -- cgit v1.2.3-70-g09d2