Messages in this thread | | | Date | Thu, 8 May 2014 12:33:33 -0400 | From | Don Zickus <> | Subject | Re: [PATCH 1/5] x86, nmi: Add new nmi type 'external' |
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On Wed, May 07, 2014 at 06:27:46PM +0200, Ingo Molnar wrote: > > [...] But I guess in theory, if a PMI NMI comes in and before the > > cpu can accept it and GHES NMI comes in, then it would suffice to > > say it may get dropped. That would be not be good. Though the race > > would be very small. > > > > I don't have a good idea how to handle that. > > Well, are GHES NMIs reasserted if they are not handled? I don't know > but there's a definite answer to that hardware behavior question.
I can't find anything that explicitly says the NMI will be re-asserted, so I will it does not. Andi, do you know? (I am not sure who maintains GHES any more).
> > > On the flip side, we have the same exact problem, today, with the > > other common external NMIs (SERR, IO). If a PCI SERR comes in at > > the same time as a PMI, then it gets dropped. Worse, it doesn't get > > re-enabled and blocks future SERRs (just found this out two weeks > > ago because of a dirty perf status register on boot). > > > > Again, I don't have a solution to juggle between PMI performance and > > reliable delivery. We could do away with the spinlocks and go back > > to single cpu delivery (like it used to be). Then devise a > > mechanism to switch delivery to another cpu upon hotplug. > > > > Thoughts? > > I'd say we should do a delayed timer that makes sure that all possible > handlers are polled after an NMI is triggered, but never at a high > rate.
Hmm, I was thinking about it and wanted to avoid a poll as I hear complaints here and there about the nmi_watchdog constantly wasting power cycles with its polling.
I was wondering if I could do a status read outside the spinlock, then if a bit is set, just grab the spin_lock and re-read the status. But then looking at the GHES code, I am not sure if it is as easy to read the status bit as it is for a PCI_SERR/IO_CHK NMI.
Andi thoughts here? Should I poke Tony Luck?
Otherwise I can set up the polling if that doesn't work.
Cheers, Don
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