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Date:	Sat, 01 Feb 2014 11:35:50 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Suresh Siddha <sbsiddha@...il.com>
CC:	Nate Eldredge <nate@...tsmathematics.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	stable <stable@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Maarten Baert <maarten-baert@...mail.com>,
	Jan Kara <jack@...e.cz>, George Spelvin <linux@...izon.com>,
	Pekka Riikonen <priikone@....fi>
Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt
 flag

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

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