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Wed, 27 Dec 2023 17:42:23 -0800 (PST) Received: from sw06.internal.sifive.com ([4.53.31.132]) by smtp.gmail.com with ESMTPSA id g24-20020aa78758000000b006d49ed3effasm7335440pfo.63.2023.12.27.17.42.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 Dec 2023 17:42:23 -0800 (PST) From: Samuel Holland To: linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-riscv@lists.infradead.org, Christoph Hellwig Cc: loongarch@lists.linux.dev, linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org, linux-arch@vger.kernel.org, Samuel Holland , Jonathan Corbet , linux-doc@vger.kernel.org Subject: [PATCH v2 01/14] arch: Add ARCH_HAS_KERNEL_FPU_SUPPORT Date: Wed, 27 Dec 2023 17:41:51 -0800 Message-ID: <20231228014220.3562640-2-samuel.holland@sifive.com> X-Mailer: git-send-email 2.42.0 In-Reply-To: <20231228014220.3562640-1-samuel.holland@sifive.com> References: <20231228014220.3562640-1-samuel.holland@sifive.com> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20231227_174225_227184_59BFFEF8 X-CRM114-Status: GOOD ( 27.86 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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. Suggested-by: Christoph Hellwig Signed-off-by: Samuel Holland Reviewed-by: Christoph Hellwig --- Changes in v2: - Add documentation explaining the built-time and runtime APIs - Add a linux/fpu.h header for generic isolation enforcement Documentation/core-api/floating-point.rst | 78 +++++++++++++++++++++++ Documentation/core-api/index.rst | 1 + Makefile | 5 ++ arch/Kconfig | 6 ++ include/linux/fpu.h | 12 ++++ 5 files changed, 102 insertions(+) create mode 100644 Documentation/core-api/floating-point.rst create mode 100644 include/linux/fpu.h 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 ======================== diff --git a/Makefile b/Makefile index ee995fc2b0e5..79c9e0b56ab8 100644 --- a/Makefile +++ b/Makefile @@ -969,6 +969,11 @@ KBUILD_CFLAGS += $(CC_FLAGS_CFI) export CC_FLAGS_CFI endif +# Architectures can define flags to add/remove for floating-point support +CC_FLAGS_FPU += -D_LINUX_FPU_COMPILATION_UNIT +export CC_FLAGS_FPU +export CC_FLAGS_NO_FPU + ifneq ($(CONFIG_FUNCTION_ALIGNMENT),0) KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT) endif diff --git a/arch/Kconfig b/arch/Kconfig index f4b210ab0612..e1c01ce819ed 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -1478,6 +1478,12 @@ config ARCH_HAS_NONLEAF_PMD_YOUNG address translations. Page table walkers that clear the accessed bit may use this capability to reduce their search space. +config ARCH_HAS_KERNEL_FPU_SUPPORT + bool + help + Architectures that select this option can run floating-point code in + the kernel, as described in Documentation/core-api/floating-point.rst. + source "kernel/gcov/Kconfig" source "scripts/gcc-plugins/Kconfig" diff --git a/include/linux/fpu.h b/include/linux/fpu.h new file mode 100644 index 000000000000..2fb63e22913b --- /dev/null +++ b/include/linux/fpu.h @@ -0,0 +1,12 @@ +/* SPDX-License-Identifier: GPL-2.0 */ + +#ifndef _LINUX_FPU_H +#define _LINUX_FPU_H + +#ifdef _LINUX_FPU_COMPILATION_UNIT +#error FP code must be compiled separately. See Documentation/core-api/floating-point.rst. +#endif + +#include + +#endif