On 2011-04-24 08:44, Avi Kivity wrote:
> On 04/24/2011 01:50 AM, OGAWA Hirofumi wrote:
>> OGAWA Hirofumi<hirofumi@xxxxxxxxxxxxxxxxxx> writes:
>>
>> > I noticed recently NMI on guest kernel is not working well.
>> host/guest
>> > kernel is 2.6.39-rc4, and using vmx.
>> >
>> > And test code is something like the following:
>> >
>> > local_irq_disable();
>> > for (i = 0; i< 10; i++) {
>> > int cpu = get_cpu();
>> > printk("%s: nmi %u, lapic %u\n", __FUNCTION__,
>> > nmi_count(cpu), irq_stat[cpu].apic_timer_irqs);
>> > mdelay(1000);
>> > put_cpu();
>> > }
>> >
>> > the result is both of nmi and lapic are not increased. If I used
>> > -no-kvm-irqchip, it works fine (increase nmi only). So, it seems to be
>> > the bug of kvm driver side.
>>
>> With some debug, the cause seems to be in pit_do_work(). With the
>> following patch, NMI watchdog seems to be working correctly (if irq
>> disabled for long time, NMI watchdog can detect it).
>>
>> Is the following patch right?
>
> This would cause IRQs to be delivered even if the PIT is masked, no?
>
> Are you in fact using the PIT? Linux prefers the HPET, and in my
> experience the -no-hpet option makes NMIs work.
BTW, that's another bug of the in-kernel PIT model: It disables the
timer in HPET legacy mode even if we are aware of NMI watchdog
receivers. Actually, the whole legacy disabling looks a bit strange in
the PIT (mode hackery + flag testing...).
While this should be fixed/refactored, adding basic perf support to KVM
will be the only option long-term as Linux dropped virtual-wire NMI
watchdog support some releases ago.
Jan
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