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

[Greg KH <gregkh@linuxfoundation.org>]

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