Re: [Qemu-devel] [PATCH for-2.9] tco: do not generate an NMI

[Laszlo Ersek]

On 04/05/17 15:49, Paolo Bonzini wrote:
> 
> 
> On 05/04/2017 15:35, Laszlo Ersek wrote:
>> Yes, I did track it back to SERR#/IOCHK#.
>>
>> What wasn't obvious to me why the TCO watchdog couldn't activate SERR#.
>>
>> I guess I missed the "SERR#" explanation in Table-2.6 "PCI Interface
>> Signals", where it is described as
>>
>>   System Error: SERR# can be pulsed active by any PCI device that
>>   detects a system error condition. Upon sampling SERR# active, the
>>   ICH9 has the ability to generate an NMI, SMI#, or interrupt.
>>
>> ... Well, I still don't see why the watchdog cannot activate SERR#. Why
>> can't it? :)
> 
> Because that's not what SERR# is for? :)  And especially because the
> effect of a watchdog timeout is described elsewhere without including
> SERR# in the picture.

That's a valid argument.

> 
>>> so that the watchdog does reboot the machine), everything works without
>>> ugly messages.
>>
>> Thanks for the explanation. I now wonder why Paulo decided to add the
>> NMI in the first place -- was he too confused by the SERR# definition?
> 
> Both Paulo and Paolo were confused. :)
> 
>>>> (Idea taken from "Documentation/sysctl/kernel.txt", near "nmi_watchdog"
>>>> -- "The NMI watchdog is disabled by default if the kernel is running as
>>>> a guest in a KVM virtual machine.")
>>>
>>> The Linux NMI watchdog is something else, it is obtained by programming
>>> a hardware performance counter on every processor.  On Linux,
>>> performance counters raise NMIs.
>>
>> Namely, for whatever reason it was wrong to raise an NMI in a KVM guest
>> through those performance counters, the same reason would be enough to
>> not raise an NMI through the iTCO_wdt driver either.
> 
> I think the reason is not that it's wrong; it's that by default KVM
> guests do not have performance counters at all.

Makes sense.

Thank you for taking the time to explain it all.

Reviewed-by: Laszlo Ersek <address@hidden>

Cheers!
Laszlo