Re: [RFC 1/3] Unified NMI delayed call mechanism

Don Zickus
2010-06-14T13:54:38Z

On Mon, Jun 14, 2010 at 12:45:21PM +0900, Hidetoshi Seto wrote:
> (2010/06/12 19:25), Ingo Molnar wrote:
> >
> > * Huang Ying <ying.huang@intel.com> wrote:
> >
> >> NMI can be triggered even when IRQ is masked. So it is not safe for NMI
> >> handler to call some functions. One solution is to delay the call via self
> >> interrupt, so that the delayed call can be done once the interrupt is
> >> enabled again. This has been implemented in MCE and perf event. This patch
> >> provides a unified version and make it easier for other NMI semantic handler
> >> to take use of the delayed call.
> >
> > Instead of introducing this extra intermediate facility please use the same
> > approach the unified NMI watchdog is using (see latest -tip): a perf event
> > callback gives all the extra functionality needed.
> >
> > The MCE code needs to be updated to use that - and then it will be integrated
> > into the events framework.
>
> Hi Ingo,
>
> I think this "NMI delayed call mechanism" could be a part of "the events
> framework" that we are planning to get in kernel soon. At least APEI will
> use NMI to report some hardware events (likely error) to kernel. So I
> suppose we will go to have a delayed call as an event handler for APEI.
>
> Generally speaking "event" can occur independently of the situation.
> NMI can tell us some of external events, expecting urgent reaction for
> the event, but we cannot do everything in NMI context. Or we might have
> a sudden urge to generate an internal event while interrupts are disabled.
>
> I agree that generating a self interrupt is reasonable solution.
> Note that it could be said that both of "MCE handled (=event log should
> be delivered to userland asap)" and "perf events pending (=pending events
> should be handled asap)" are kind of internal event that requires urgent
> handling in non-NMI kernel context. One question here is why we should
> have different vectors for these events that uses same mechanism.

I think the perf event subsytem can log events in NMI context already and
deliver them to userspace when the NMI is done. This is why I think Ingo
wants MCE to be updated to sit on top of the perf event subsytem to avoid
re-invent everything again.

Then again I do not know enough about the MCE stuff to understand what you
mean when an event comes in but you can't handle it in an NMI-safe
context. An example would be helpful.

Cheers,
Don