Message-ID: <48f508aa-ab40-7032-a68d-90d8986afb2f@xen0n.name>
Date:   Sun, 5 Mar 2023 13:53:21 +0800
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Subject: Re: [PATCH] LoongArch: Provide kernel fpu functions
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To:     Huacai Chen <chenhuacai@loongson.cn>,
        Arnd Bergmann <arnd@arndb.de>,
        Huacai Chen <chenhuacai@kernel.org>
Cc:     loongarch@lists.linux.dev, linux-arch@vger.kernel.org,
        Xuefeng Li <lixuefeng@loongson.cn>,
        Guo Ren <guoren@kernel.org>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        linux-kernel@vger.kernel.org, loongson-kernel@lists.loongnix.cn
References: <20230305052818.4030447-1-chenhuacai@loongson.cn>
From:   WANG Xuerui <kernel@xen0n.name>
In-Reply-To: <20230305052818.4030447-1-chenhuacai@loongson.cn>
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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@xen0n.name>
Thanks, but I prefer my name spelled in the native word order ;-)
> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---
>   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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