Re: [PATCH] LoongArch: Provide kernel fpu functions

From: WANG Xuerui
Date: Sun Mar 05 2023 - 00:53:38 EST


On 3/5/23 13:28, Huacai Chen wrote:
Provide kernel_fpu_begin()/kernel_fpu_end() to let the kernel use fpu
itself. They can be used by AMDGPU graphic driver for DCN.

Grammar nit: "itself" is wrongly placed. "allow the kernel itself to use FPU" could be better.

Also the expected usage is way broader than a single driver's single component. It's useful for a wide array of operations that will benefit from SIMD acceleration support that'll hopefully appear later. For now I'd suggest at least adding a single "e.g." after "used by" to signify this, if you're not rewording the sentence.


Reported-by: Xuerui Wang <kernel@xxxxxxxxxx>
Thanks, but I prefer my name spelled in the native word order ;-)
Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
---
arch/loongarch/include/asm/fpu.h | 3 +++
arch/loongarch/kernel/Makefile | 2 +-
arch/loongarch/kernel/kfpu.c | 41 ++++++++++++++++++++++++++++++++
3 files changed, 45 insertions(+), 1 deletion(-)
create mode 100644 arch/loongarch/kernel/kfpu.c

diff --git a/arch/loongarch/include/asm/fpu.h b/arch/loongarch/include/asm/fpu.h
index 358b254d9c1d..192f8e35d912 100644
--- a/arch/loongarch/include/asm/fpu.h
+++ b/arch/loongarch/include/asm/fpu.h
@@ -21,6 +21,9 @@
struct sigcontext;
+extern void kernel_fpu_begin(void);
+extern void kernel_fpu_end(void);
+
extern void _init_fpu(unsigned int);
extern void _save_fp(struct loongarch_fpu *);
extern void _restore_fp(struct loongarch_fpu *);
diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
index 78d4e3384305..9a72d91cd104 100644
--- a/arch/loongarch/kernel/Makefile
+++ b/arch/loongarch/kernel/Makefile
@@ -13,7 +13,7 @@ obj-y += head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
obj-$(CONFIG_ACPI) += acpi.o
obj-$(CONFIG_EFI) += efi.o
-obj-$(CONFIG_CPU_HAS_FPU) += fpu.o
+obj-$(CONFIG_CPU_HAS_FPU) += fpu.o kfpu.o
obj-$(CONFIG_ARCH_STRICT_ALIGN) += unaligned.o
diff --git a/arch/loongarch/kernel/kfpu.c b/arch/loongarch/kernel/kfpu.c
new file mode 100644
index 000000000000..cd2a18fecdcc
--- /dev/null
+++ b/arch/loongarch/kernel/kfpu.c
@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2020-2023 Loongson Technology Corporation Limited
+ */
+
+#include <linux/cpu.h>
+#include <linux/init.h>
+#include <asm/fpu.h>
+#include <asm/smp.h>
+
+static DEFINE_PER_CPU(bool, in_kernel_fpu);
+
+void kernel_fpu_begin(void)
+{
+ if(this_cpu_read(in_kernel_fpu))
+ return;
Could be a conditional WARN_ON_ONCE like in arch/x86?
+
+ preempt_disable();
+ this_cpu_write(in_kernel_fpu, true);
+
+ if (!is_fpu_owner())
+ enable_fpu();
+ else
+ _save_fp(&current->thread.fpu);
+}
+EXPORT_SYMBOL_GPL(kernel_fpu_begin);

Might be good to provide some explanation in the commit message as to why the pair of helpers should be GPL-only. Do they touch state buried deep enough to make any downstream user a "derivative work"? Or are the annotation inspired by arch/x86?

I think this kinda needs more thought, because similar operations like arm's kernel_neon_{begin,end}, powerpc's enable_kernel_{fp,vsx,altivec} or s390's __kernel_fpu_{begin,end} are not made GPL-only. Making these helpers GPL-only precludes any non-GPL module to make use of SIMD on LoongArch, which may or may not be what you want. This can have commercial consequences so I can only leave the decision to you. (Although IMO the semantics are encapsulated and high-level enough to not warrant GPL-only marks, but it may well be the case that you have thought of something else but didn't mention here.)

+
+void kernel_fpu_end(void)
+{
+ if(!this_cpu_read(in_kernel_fpu))
+ return;
+
+ if (!is_fpu_owner())
+ disable_fpu();
+ else
+ _restore_fp(&current->thread.fpu);
+
+ this_cpu_write(in_kernel_fpu, false);
+ preempt_enable();
+}
+EXPORT_SYMBOL_GPL(kernel_fpu_end);

--
WANG "xen0n" Xuerui

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