linux-kernel - Re: [tip:x86/urgent] x86, fpu: correct the asm constraints for fxsave , unbreak mxcsr.daz

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Message-ID: <20130726202512.GA4585@kroah.com>
Date:	Fri, 26 Jul 2013 13:25:12 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-stable <stable@...r.kernel.org>, hpa@...ux.intel.com,
	hjl.tools@...il.com, mingo@...nel.org, tglx@...utronix.de,
	"tip-bot for H.J. Lu" <tipbot@...or.com>,
	linux-tip-commits@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [tip:x86/urgent] x86, fpu: correct the asm constraints for
 fxsave , unbreak mxcsr.daz

On Fri, Jul 26, 2013 at 01:19:30PM -0700, H. Peter Anvin wrote:
> Hi everyone,
> 
> I have tagged this one for -stable, but it is a definite "maybe", so I
> wanted to:
> 
> a) qualify that tagging, and
> b) perhaps use it as a boundary case for policy clarification.
> 
> This patch is an old bug, an incorrect assembly constraint, but as
> happens with this type of bug it is being exposed by a change in gcc.
> Therefore, although this is neither a regression per se in the kernel
> nor in gcc, it is a regression in the combined output.
> 
> The bug is obvious; the fix at the very least *should* be obvious; the
> main source of risk would appear that it might trigger bugs in very old
> versions of gcc.
> 
> The gain is less inherently obvious: currently the only MXCSR bit
> affected is DAZ, although this also affects any future MXCSR bits.  Most
> users do not care about DAZ, but the ones that do generally care *a lot*
> about it as their applications may suffer very bad performance otherwise.
> 
> There is a kernel bugzilla for this:
> 
> 	https://bugzilla.kernel.org/show_bug.cgi?id=60633
> 
> ... but it doesn't really add any additional information.

Thanks for the additional information, it's much appreciated.

greg k-h
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