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 }