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  */
fpu__init_check_bugs(void)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 }
60