arch/x86/kernel/fpu/bugs.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * x86 FPU bug checks:
   4  */
   5 #include <asm/fpu/internal.h>
   6
   7 /*
   8  * Boot time CPU/FPU FDIV bug detection code:
   9  */
  10
  11 static double __initdata x = 4195835.0;
  12 static double __initdata y = 3145727.0;
  13
  14 /*
  15  * This used to check for exceptions..
  16  * However, it turns out that to support that,
  17  * the XMM trap handlers basically had to
  18  * be buggy. So let's have a correct XMM trap
  19  * handler, and forget about printing out
  20  * some status at boot.
  21  *
  22  * We should really only care about bugs here
  23  * anyway. Not features.
  24  */
  25 void __init fpu__init_check_bugs(void)
  26 {
  27         s32 fdiv_bug;
  28
  29         /* kernel_fpu_begin/end() relies on patched alternative instructions. */
  30         if (!boot_cpu_has(X86_FEATURE_FPU))
  31                 return;
  32
  33         kernel_fpu_begin();
  34
  35         /*
  36          * trap_init() enabled FXSR and company _before_ testing for FP
  37          * problems here.
  38          *
  39          * Test for the divl bug: http://en.wikipedia.org/wiki/Fdiv_bug
  40          */
  41         __asm__("fninit\n\t"
  42                 "fldl %1\n\t"
  43                 "fdivl %2\n\t"
  44                 "fmull %2\n\t"
  45                 "fldl %1\n\t"
  46                 "fsubp %%st,%%st(1)\n\t"
  47                 "fistpl %0\n\t"
  48                 "fwait\n\t"
  49                 "fninit"
  50                 : "=m" (*&fdiv_bug)
  51                 : "m" (*&x), "m" (*&y));
  52
  53         kernel_fpu_end();
  54
  55         if (fdiv_bug) {
  56                 set_cpu_bug(&boot_cpu_data, X86_BUG_FDIV);
  57                 pr_warn("Hmm, FPU with FDIV bug\n");
  58         }
  59 }