1// SPDX-License-Identifier: GPL-2.0
 2/*
 3 * x86 FPU bug checks:
 4 */
 5#include <linux/printk.h>
 6
 7#include <asm/cpufeature.h>
 8#include <asm/fpu/api.h>
 9
10/*
11 * Boot time CPU/FPU FDIV bug detection code:
12 */
13
14static double __initdata x = 4195835.0;
15static double __initdata y = 3145727.0;
16
17/*
18 * This used to check for exceptions..
19 * However, it turns out that to support that,
20 * the XMM trap handlers basically had to
21 * be buggy. So let's have a correct XMM trap
22 * handler, and forget about printing out
23 * some status at boot.
24 *
25 * We should really only care about bugs here
26 * anyway. Not features.
27 */
28void __init fpu__init_check_bugs(void)
29{
30	s32 fdiv_bug;
31
32	/* kernel_fpu_begin/end() relies on patched alternative instructions. */
33	if (!boot_cpu_has(X86_FEATURE_FPU))
34		return;
35
36	kernel_fpu_begin();
37
38	/*
39	 * trap_init() enabled FXSR and company _before_ testing for FP
40	 * problems here.
41	 *
42	 * Test for the divl bug: http://en.wikipedia.org/wiki/Fdiv_bug
43	 */
44	__asm__("fninit\n\t"
45		"fldl %1\n\t"
46		"fdivl %2\n\t"
47		"fmull %2\n\t"
48		"fldl %1\n\t"
49		"fsubp %%st,%%st(1)\n\t"
50		"fistpl %0\n\t"
51		"fwait\n\t"
52		"fninit"
53		: "=m" (*&fdiv_bug)
54		: "m" (*&x), "m" (*&y));
55
56	kernel_fpu_end();
57
58	if (fdiv_bug) {
59		set_cpu_bug(&boot_cpu_data, X86_BUG_FDIV);
60		pr_warn("Hmm, FPU with FDIV bug\n");
61	}
62}