What reassert_nmi() in default_do_nmi() is for?

What reassert_nmi() in default_do_nmi() is for?

[Huang Ying]

Hi, All,

When reading NMI handler code, I find there is reassert_nmi() in the end
of default_do_nmi(). And it is only called if the reason & 0xc0 != 0
(reason = inb(0x61)).

The comments for it is as follow:

	/*
	 * Reassert NMI in case it became active meanwhile
	 * as it's edge-triggered:
	 */
	reassert_nmi();

The implementation of reassert_nmi is as follow:

static inline void reassert_nmi(void)
{
        int old_reg = -1;

        if (do_i_have_lock_cmos())
                old_reg = current_lock_cmos_reg();
        else
                lock_cmos(0); /* register doesn't matter here */
        outb(0x8f, 0x70);
        inb(0x71);              /* dummy */
        outb(0x0f, 0x70);
        inb(0x71);              /* dummy */
        if (old_reg >= 0)
                outb(old_reg, 0x70);
        else
                unlock_cmos();
}

I can not understand the code and the logic. Can anyone explain it in
more detail?

Thanks,
Huang Ying

Re: What reassert_nmi() in default_do_nmi() is for?

[Paolo Cancedda]

> The implementation of reassert_nmi is as follow:
>
> [...]
>
> I can not understand the code and the logic. Can anyone explain it in
> more detail?

It's there since 2.3.20 (!), see
http://www.tm.kernel.org/pub/linux/kernel/v2.3/patch-html/patch-2.3.20/linux_arch_i386_kernel_traps.c.html
Googling around I found this thread:
http://www.gossamer-threads.com/lists/linux/kernel/140216
HTH!
Cheers

P