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Date:   Wed, 20 Jan 2021 11:07:11 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     x86@kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Krzysztof Mazur <krzysiek@podlesie.net>,
        Krzysztof =?utf-8?Q?Ol=C4=99dzki?= <ole@ans.pl>,
        Arnd Bergmann <arnd@arndb.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH v2 4/4] x86/fpu/64: Don't FNINIT in kernel_fpu_begin()
Message-ID: <YAgAz4qYESIv8iNR@hirez.programming.kicks-ass.net>
References: <cover.1611077835.git.luto@kernel.org>
 <803d45a172a25314fdaec0a01aada8333d55df4e.1611077835.git.luto@kernel.org>
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On Tue, Jan 19, 2021 at 09:39:02AM -0800, Andy Lutomirski wrote:
> The remaining callers of kernel_fpu_begin() in 64-bit kernels don't use 387
> instructions, so there's no need to sanitize the FPU state.  Skip it to get
> most of the performance we lost back.
> 
> Reported-by: Krzysztof Olędzki <ole@ans.pl>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/x86/include/asm/fpu/api.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
> index 38f4936045ab..435bc59d539b 100644
> --- a/arch/x86/include/asm/fpu/api.h
> +++ b/arch/x86/include/asm/fpu/api.h
> @@ -32,7 +32,19 @@ extern void fpregs_mark_activate(void);
>  /* Code that is unaware of kernel_fpu_begin_mask() can use this */
>  static inline void kernel_fpu_begin(void)
>  {
> +#ifdef CONFIG_X86_64
> +	/*
> +	 * Any 64-bit code that uses 387 instructions must explicitly request
> +	 * KFPU_387.
> +	 */
> +	kernel_fpu_begin_mask(KFPU_MXCSR);

I'm also still sitting on this:

  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/fpu

what do we do with that?