linux-kernel - Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

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Date:	Fri, 20 Feb 2015 20:05:54 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Oleg Nesterov <oleg@...hat.com>, Rik van Riel <riel@...hat.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

+ Linus.

I'm sure he'll save something to say about it :-)

On Fri, Feb 20, 2015 at 10:58:15AM -0800, Andy Lutomirski wrote:
> We have eager and lazy fpu modes, introduced in:
> 
> 304bceda6a18 x86, fpu: use non-lazy fpu restore for processors supporting xsave
> 
> The result is rather messy.  There are two code paths in almost all of the
> FPU code, and only one of them (the eager case) is tested frequently, since
> most kernel developers have new enough hardware that we use eagerfpu.
> 
> It seems that, on any remotely recent hardware, eagerfpu is a win:
> glibc uses SSE2, so laziness is probably overoptimistic, and, in any
> case, manipulating TS is far slower that saving and restoring the full
> state.
> 
> To try to shake out any latent issues on old hardware, this changes
> the default to eager on all CPUs.  If no performance or functionality
> problems show up, a subsequent patch could remove lazy mode entirely.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
>  arch/x86/kernel/xsave.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
> index 0de1fae2bdf0..1928c8f34ce5 100644
> --- a/arch/x86/kernel/xsave.c
> +++ b/arch/x86/kernel/xsave.c
> @@ -637,8 +637,8 @@ static void __init xstate_enable_boot_cpu(void)
>  	prepare_fx_sw_frame();
>  	setup_init_fpu_buf();
>  
> -	/* Auto enable eagerfpu for xsaveopt */
> -	if (cpu_has_xsaveopt && eagerfpu != DISABLE)
> +	/* Auto enable eagerfpu for everyone */
> +	if (eagerfpu != DISABLE)
>  		eagerfpu = ENABLE;
>  
>  	if (pcntxt_mask & XSTATE_EAGER) {
> -- 
> 2.1.0
> 
> 

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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