linux-kernel - Re: [PATCH V3] LoongArch: Provide kernel fpu functions

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Date:   Mon, 6 Mar 2023 20:03:29 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, linux-arch@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn
Subject: Re: [PATCH V3] LoongArch: Provide kernel fpu functions



在 2023/3/6 17:59, Huacai Chen 写道:
> Provide kernel_fpu_begin()/kernel_fpu_end() to allow the kernel itself
> to use fpu. They can be used by some other kernel components, e.g., the
> AMDGPU graphic driver for DCN.
Since kernel is compiled with -msoft-float, I guess hw fpu will not be
used in kernel by present:). However it is deserved to try.
> 
> Reported-by: WANG Xuerui <kernel@...0n.name>
> Tested-by: WANG Xuerui <kernel@...0n.name>
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
> V2: Use non-GPL exports and update commit messages.
> V3: Add spaces for coding style.
> 
>  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) 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;
> +
> +	preempt_disable();
> +	this_cpu_write(in_kernel_fpu, true);
> +
> +	if (!is_fpu_owner())
> +		enable_fpu();
> +	else
> +		_save_fp(&current->thread.fpu);
Do we need initialize fcsr rather than using random fcsr value
of other processes? There may be fpu exception enabled by
other tasks.

Regards
Bibo,mao
> +}
> +EXPORT_SYMBOL(kernel_fpu_begin);
> +
> +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(kernel_fpu_end);