Re: nmi is broken?

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-04-24 08:44, Avi Kivity wrote:
> On 04/24/2011 01:50 AM, OGAWA Hirofumi wrote:
>> OGAWA Hirofumi<hirofumi@mail.parknet.co.jp>  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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