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

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Date:   Mon, 6 Mar 2023 16:57:57 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Huacai Chen' <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>,
        Huacai Chen <chenhuacai@...nel.org>
CC:     "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
        "linux-arch@...r.kernel.org" <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" <linux-kernel@...r.kernel.org>,
        "loongson-kernel@...ts.loongnix.cn" 
        <loongson-kernel@...ts.loongnix.cn>
Subject: RE: [PATCH V3] LoongArch: Provide kernel fpu functions

From: Huacai Chen
> Sent: 06 March 2023 10:00
> 
> 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.
> 
...
> +void kernel_fpu_end(void)
> +{
> +	if (!this_cpu_read(in_kernel_fpu))
> +		return;

Shouldn't it be better for that to be a counter?
Not sure what anyone else does.

> +
> +	if (!is_fpu_owner())
> +		disable_fpu();
> +	else
> +		_restore_fp(&current->thread.fpu);

Does that actual do the restore?
You really don't need to do it until 'return to user'.
That will speed up a subsequent kernel_fpu_begin().

If fact, couldn't kernel_fpu_end() just be preemt_enable()?

	David

> +
> +	this_cpu_write(in_kernel_fpu, false);
> +	preempt_enable();
> +}
> +EXPORT_SYMBOL(kernel_fpu_end);
> --
> 2.39.1

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