linux-kernel - [RFC PATCH 01/12] arch: Add ARCH_HAS_KERNEL_FPU

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Date:   Thu,  7 Dec 2023 21:54:31 -0800
From:   Samuel Holland <samuel.holland@...ive.com>
To:     linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-riscv@...ts.infradead.org,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        linux-arch@...r.kernel.org,
        Samuel Holland <samuel.holland@...ive.com>
Subject: [RFC PATCH 01/12] arch: Add ARCH_HAS_KERNEL_FPU_SUPPORT

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 <hch@...radead.org>
Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
---

 Makefile     | 4 ++++
 arch/Kconfig | 9 +++++++++
 2 files changed, 13 insertions(+)

diff --git a/Makefile b/Makefile
index 511b5616aa41..e65c186cf2c9 100644
--- a/Makefile
+++ b/Makefile
@@ -969,6 +969,10 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_CFI)
 export CC_FLAGS_CFI
 endif
 
+# Architectures can define flags to add/remove for floating-point support
+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..6df834e18e9c 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1478,6 +1478,15 @@ 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
+	  An architecture should select this option if it supports running
+	  floating-point code in kernel space. It must export the functions
+	  kernel_fpu_available(), kernel_fpu_begin(), and kernel_fpu_end() from
+	  <asm/fpu.h>, and define CC_FLAGS_FPU and/or CC_FLAGS_NO_FPU as
+	  necessary in its Makefile.
+
 source "kernel/gcov/Kconfig"
 
 source "scripts/gcc-plugins/Kconfig"
-- 
2.42.0