Messages in this thread | | | Date | Fri, 26 Jul 2013 13:19:30 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [tip:x86/urgent] x86, fpu: correct the asm constraints for fxsave , unbreak mxcsr.daz |
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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.
-hpa
On 07/26/2013 09:19 AM, tip-bot for H.J. Lu wrote: > Commit-ID: eaa5a990191d204ba0f9d35dbe5505ec2cdd1460 > Gitweb: http://git.kernel.org/tip/eaa5a990191d204ba0f9d35dbe5505ec2cdd1460 > Author: H.J. Lu <hjl.tools@gmail.com> > AuthorDate: Fri, 26 Jul 2013 09:11:56 -0700 > Committer: H. Peter Anvin <hpa@linux.intel.com> > CommitDate: Fri, 26 Jul 2013 09:11:56 -0700 > > x86, fpu: correct the asm constraints for fxsave, unbreak mxcsr.daz > > GCC will optimize mxcsr_feature_mask_init in arch/x86/kernel/i387.c: > > memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct)); > asm volatile("fxsave %0" : : "m" (fx_scratch)); > mask = fx_scratch.mxcsr_mask; > if (mask == 0) > mask = 0x0000ffbf; > > to > > memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct)); > asm volatile("fxsave %0" : : "m" (fx_scratch)); > mask = 0x0000ffbf; > > since asm statement doesn’t say it will update fx_scratch. As the > result, the DAZ bit will be cleared. This patch fixes it. This bug > dates back to at least kernel 2.6.12. > > Signed-off-by: H. Peter Anvin <hpa@linux.intel.com> > Cc: <stable@vger.kernel.org> > --- > arch/x86/kernel/i387.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c > index 202d24f..5d576ab 100644 > --- a/arch/x86/kernel/i387.c > +++ b/arch/x86/kernel/i387.c > @@ -116,7 +116,7 @@ static void mxcsr_feature_mask_init(void) > > if (cpu_has_fxsr) { > memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct)); > - asm volatile("fxsave %0" : : "m" (fx_scratch)); > + asm volatile("fxsave %0" : "+m" (fx_scratch)); > mask = fx_scratch.mxcsr_mask; > if (mask == 0) > mask = 0x0000ffbf; >
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