linux-kernel - Re: [PATCH 03/22] x86/fpu: Remove preempt_disable() in fpu_

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Message-ID: <20190114185506.GP2773@zn.tnic>
Date:   Mon, 14 Jan 2019 19:55:06 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 03/22] x86/fpu: Remove preempt_disable() in fpu__clear()

On Wed, Jan 09, 2019 at 12:47:25PM +0100, Sebastian Andrzej Siewior wrote:
> The preempt_disable() section was introduced in commit

<---- newline here.

>   a10b6a16cdad8 ("x86/fpu: Make the fpu state change in fpu__clear() scheduler-atomic")

<---- newline here.

> and it was said to be temporary.
> 
> fpu__initialize() initializes the FPU struct to its "init" value and
> then sets ->initialized to 1. The last part is the important one.
> The content of the `state' does not matter because it gets set via
> copy_init_fpstate_to_fpregs().
> A preemption here has little meaning because the register will always be

s/register/registers/

> set to the same content after copy_init_fpstate_to_fpregs(). A softirq
> with a kernel_fpu_begin() could also force to save FPU's register after

ditto.

> fpu__initialize() without changing the outcome here.
> 
> Remove the preempt_disable() section in fpu__clear(), preemption here
> does not hurt.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  arch/x86/kernel/fpu/core.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 1d3ae7988f7f2..1940319268aef 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -366,11 +366,9 @@ void fpu__clear(struct fpu *fpu)
>  	 * Make sure fpstate is cleared and initialized.
>  	 */
>  	if (static_cpu_has(X86_FEATURE_FPU)) {
> -		preempt_disable();
>  		fpu__initialize(fpu);
>  		user_fpu_begin();
>  		copy_init_fpstate_to_fpregs();
> -		preempt_enable();
>  	}
>  }
>  
> -- 

With the above addressed:

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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