linux-kernel - Re: [PATCH 3/4] x86/fpu: use restore_init_xstate() instead of math_state

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Message-ID: <20150313104801.GC31998@pd.tnic>
Date:	Fri, 13 Mar 2015 11:48:01 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH 3/4] x86/fpu: use restore_init_xstate() instead of
 math_state_restore() on kthread exec

On Wed, Mar 11, 2015 at 06:34:49PM +0100, Oleg Nesterov wrote:
> Change flush_thread() to do user_fpu_begin() + restore_init_xstate()
> and avoid math_state_restore().
> 
> Note: "TODO: cleanup this horror" is still valid. We do not need
> init_fpu() at all, we only need fpu_alloc() + memset(0). But this needs
> other changes, in particular user_fpu_begin() should set used_math().
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  arch/x86/kernel/process.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index dd9a069..c396de2 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -142,7 +142,8 @@ void flush_thread(void)
>  		/* kthread execs. TODO: cleanup this horror. */
>  		if (WARN_ON(init_fpu(current)))
>  			force_sig(SIGKILL, current);
> -		math_state_restore();
> +		user_fpu_begin();
> +		restore_init_xstate();

Ok, question: so math_state_restore() does kernel_fpu_disable() before
doing those, why is it ok for flush_thread() to not do it?

-- 
Regards/Gruss,
    Boris.

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