ARM Cortex A8 PMNC 读取在启用后也给出 0.. 有什么想法/建议吗？

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("user-mode access to performance registers");

int __init arm_init(void)

{
    unsigned int value;

    /* enable user-mode access */
    printk(KERN_INFO "enable user-mode access\n");
    asm ("MCR p15, 0, %0, C9, C14, 0\n\t" :: "r"(1));

    /* Reading the value here--just to check */

    asm ("MRC p15, 0, %0, c9, c14, 0\t\n": "=r"(value));
    printk("value: %d\n", value);


    /* disable counter overflow interrupts (just in case)*/
    printk(KERN_INFO "disable counter overflow interrupts (just in case)\n");
    asm ("MCR p15, 0, %0, C9, C14, 2\n\t" :: "r"(0x8000000f));

    printk(KERN_INFO "user-mode access to performance registers enabled\n");
    return 0;
}


void arm_exit(void)
{
    unsigned int value;
    asm ("MRC p15, 0, %0, c9, c14, 0\t\n": "=r"(value));
    printk("value: %d\n", value);
    printk(KERN_INFO "user-mode access to performance registers disabled\n");
}

module_init(arm_init);
module_exit(arm_exit);

在 init 模块中读取给出 1，但在 cleanup 模块中读取变量给出 0。 知道它如何更新吗？