arch/x86/kernel/fpu/bugs.c

   1 /*
   2  * x86 FPU bug checks:
   3  */
   4 #include <asm/fpu/internal.h>
   5
   6 /*
   7  * Boot time CPU/FPU FDIV bug detection code:
   8  */
   9
  10 static double __initdata x = 4195835.0;
  11 static double __initdata y = 3145727.0;
  12
  13 /*
  14  * This used to check for exceptions..
  15  * However, it turns out that to support that,
  16  * the XMM trap handlers basically had to
  17  * be buggy. So let's have a correct XMM trap
  18  * handler, and forget about printing out
  19  * some status at boot.
  20  *
  21  * We should really only care about bugs here
  22  * anyway. Not features.
  23  */
  24 static void __init check_fpu(void)
  25 {
  26         u32 cr0_saved;
  27         s32 fdiv_bug;
  28
  29         /* We might have CR0::TS set already, clear it: */
  30         cr0_saved = read_cr0();
  31         write_cr0(cr0_saved & ~X86_CR0_TS);
  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         write_cr0(cr0_saved);
  56
  57         if (fdiv_bug) {
  58                 set_cpu_bug(&boot_cpu_data, X86_BUG_FDIV);
  59                 pr_warn("Hmm, FPU with FDIV bug\n");
  60         }
  61 }
  62
  63 void __init fpu__init_check_bugs(void)
  64 {
  65         /*
  66          * kernel_fpu_begin/end() in check_fpu() relies on the patched
  67          * alternative instructions.
  68          */
  69         if (cpu_has_fpu)
  70                 check_fpu();
  71 }