releases/3.0.90/x86-fpu-correct-the-asm-constraints-for-fxsave-unbreak-mxcsr.daz.patch

   1 From eaa5a990191d204ba0f9d35dbe5505ec2cdd1460 Mon Sep 17 00:00:00 2001
   2 From: "H.J. Lu" <hjl.tools@gmail.com>
   3 Date: Fri, 26 Jul 2013 09:11:56 -0700
   4 Subject: x86, fpu: correct the asm constraints for fxsave, unbreak mxcsr.daz
   5
   6 From: "H.J. Lu" <hjl.tools@gmail.com>
   7
   8 commit eaa5a990191d204ba0f9d35dbe5505ec2cdd1460 upstream.
   9
  10 GCC will optimize mxcsr_feature_mask_init in arch/x86/kernel/i387.c:
  11
  12                 memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct));
  13                 asm volatile("fxsave %0" : : "m" (fx_scratch));
  14                 mask = fx_scratch.mxcsr_mask;
  15                 if (mask == 0)
  16                         mask = 0x0000ffbf;
  17
  18 to
  19
  20                 memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct));
  21                 asm volatile("fxsave %0" : : "m" (fx_scratch));
  22                 mask = 0x0000ffbf;
  23
  24 since asm statement doesn’t say it will update fx_scratch.  As the
  25 result, the DAZ bit will be cleared.  This patch fixes it. This bug
  26 dates back to at least kernel 2.6.12.
  27
  28 Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
  29 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  30
  31 ---
  32  arch/x86/kernel/i387.c |    2 +-
  33  1 file changed, 1 insertion(+), 1 deletion(-)
  34
  35 --- a/arch/x86/kernel/i387.c
  36 +++ b/arch/x86/kernel/i387.c
  37 @@ -51,7 +51,7 @@ void __cpuinit mxcsr_feature_mask_init(v
  38         clts();
  39         if (cpu_has_fxsr) {
  40                 memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct));
  41 -               asm volatile("fxsave %0" : : "m" (fx_scratch));
  42 +               asm volatile("fxsave %0" : "+m" (fx_scratch));
  43                 mask = fx_scratch.mxcsr_mask;
  44                 if (mask == 0)
  45                         mask = 0x0000ffbf;