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Date: Sat, 01 Feb 2014 11:35:50 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>,
	Suresh Siddha <sbsiddha@gmail.com>
CC: Nate Eldredge <nate@thatsmathematics.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
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	stable <stable@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Maarten Baert <maarten-baert@hotmail.com>,
	Jan Kara <jack@suse.cz>, George Spelvin <linux@horizon.com>,
	Pekka Riikonen <priikone@iki.fi>
Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt
 flag
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On 02/01/2014 11:27 AM, Linus Torvalds wrote:
>
>   (a) "we don't want to restore at all, because once the kernel starts
> using math, it might do so a lot, and saving/restoring is a bad idea":
>
>    void __kernel_fpu_end(void)
>    {
>            stts();
>    }
>
> *or*
>
> Quite frankly, I'd almost lean towards (a). Comments? Does anybody
> have any loads where the kernel does a lot of fpu stuff (ie network
> encryption using the hw engines or something)? I'd really like to hear
> if it makes a difference..
>

This will obviously not protect eageronly features (MPX, LWP, ...) so 
this means those features are permanently unavailable to the kernel, 
even inside kernel_fpu_begin/end.  Now, currently I don't think we have 
any plans to use those in the kernel (at least not in a way where 
kernel_fpu_begin/end makes sense as bracketing), but it is something 
worth keeping in mind.

	-hpa