1// SPDX-License-Identifier: GPL-2.0
 2/*
 3 * x86 FPU bug checks:
 4 */
 5#include <asm/cpufeature.h>
 6#include <asm/fpu/api.h>
 7
 8/*
 9 * Boot time CPU/FPU FDIV bug detection code:
10 */
11
12static double __initdata x = 4195835.0;
13static double __initdata y = 3145727.0;
14
15/*
16 * This used to check for exceptions..
17 * However, it turns out that to support that,
18 * the XMM trap handlers basically had to
19 * be buggy. So let's have a correct XMM trap
20 * handler, and forget about printing out
21 * some status at boot.
22 *
23 * We should really only care about bugs here
24 * anyway. Not features.
25 */
26void __init fpu__init_check_bugs(void)
27{
28	s32 fdiv_bug;
29
30	/* kernel_fpu_begin/end() relies on patched alternative instructions. */
31	if (!boot_cpu_has(X86_FEATURE_FPU))
32		return;
33
34	kernel_fpu_begin();
35
36	/*
37	 * trap_init() enabled FXSR and company _before_ testing for FP
38	 * problems here.
39	 *
40	 * Test for the divl bug: http://en.wikipedia.org/wiki/Fdiv_bug
41	 */
42	__asm__("fninit\n\t"
43		"fldl %1\n\t"
44		"fdivl %2\n\t"
45		"fmull %2\n\t"
46		"fldl %1\n\t"
47		"fsubp %%st,%%st(1)\n\t"
48		"fistpl %0\n\t"
49		"fwait\n\t"
50		"fninit"
51		: "=m" (*&fdiv_bug)
52		: "m" (*&x), "m" (*&y));
53
54	kernel_fpu_end();
55
56	if (fdiv_bug) {
57		set_cpu_bug(&boot_cpu_data, X86_BUG_FDIV);
58		pr_warn("Hmm, FPU with FDIV bug\n");
59	}
60}