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
14 static double __initdata x = 4195835.0;
15 static 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 */
28 void __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 }
63
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