linux-kernel - Re: [PATCH 1/5] x86/fpu: Fix math emulation in eager fpu mode

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Message-ID: <20160123100253.GA16329@pd.tnic>
Date:	Sat, 23 Jan 2016 11:02:54 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Fenghua Yu <fenghua.yu@...el.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	yu-cheng yu <yu-cheng.yu@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/5] x86/fpu: Fix math emulation in eager fpu mode

On Fri, Jan 22, 2016 at 04:56:02PM -0800, Andy Lutomirski wrote:
> Systems without an FPU are generally old and therefore use lazy FPU
> switching.  Unsurprisingly, math emulation in eager FPU mode is a
> bit buggy.  Fix it.
> 
> There were two bugs involving kernel code trying to use the FPU
> registers in eager mode even if they didn't exist and one BUG_ON
> that was incorrect.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/include/asm/fpu/internal.h | 3 ++-
>  arch/x86/kernel/fpu/core.c          | 2 +-
>  arch/x86/kernel/traps.c             | 1 -
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> index 0fd440df63f1..a1f78a9fbf41 100644
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -589,7 +589,8 @@ switch_fpu_prepare(struct fpu *old_fpu, struct fpu *new_fpu, int cpu)
>  	 * If the task has used the math, pre-load the FPU on xsave processors
>  	 * or if the past 5 consecutive context-switches used math.
>  	 */
> -	fpu.preload = new_fpu->fpstate_active &&
> +	fpu.preload = static_cpu_has(X86_FEATURE_FPU) &&
> +		      new_fpu->fpstate_active &&
>  		      (use_eager_fpu() || new_fpu->counter > 5);

Should we move that static_cpu_has(X86_FEATURE_FPU) check in
use_eager_fpu()?

I mean, when !X86_FEATURE_FPU, then we most certainly aren't doing eager
FPU anyway.

Looking at the call sites briefly says we should be covered but I might
be missing out some detail.

-- 
Regards/Gruss,
    Boris.

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