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From: Mathieu Desnoyers on 13 Apr 2010 13:20
* Masami Hiramatsu (mhiramat(a)redhat.com) wrote:
> Mathieu Desnoyers wrote:
> >> It is *not* necessary to wait for the breakpoint handlers to return, as
> >> long as they will get to IRET eventually, since IRET is a jump and a
> >> serializing instruction.
> >
> > Ah, I see. So the added smp_mb() would not be needed then, as long as we
> > know that the other CPUs either are currently running the IPI handler or
> > have executed it. IOW: they will execute IRET very soon or they just
> > executed it since the int3 have been written. I am a bit concerned about
> > NMIs coming in this race window, but as they need to have started after
> > we have put the breakpoint, that should be OK. (note: entry_*.S
> > modifications are needed to support nesting breakpoint handlers in NMIs)
>
> Hmm, if we support this to modify NMI code, it seems that we need to
> support not only nesting breakpoint handling but also nesting NMIs,
> because nesting NMI is unblocked when next IRET (of breakpoint) is
> issued.
>
> From Intel's Software Developer's Manual Vol.3A 5.7.1 Handling Multiple NMIs
> said below.
> ---
> While an NMI interrupt handler is executing, the processor disables additional calls to
> the NMI handler until the next IRET instruction is executed. This blocking of subse-
> quent NMIs prevents stacking up calls to the NMI handler. [...]
> ---
>
> I assume that your below patch tried to solve this issue, right?
> http://lkml.indiana.edu/hypermail/linux/kernel/0804.1/0965.html
>

Yep. (sorry about late reply).

Mathieu

> Thank you,
>
> --
> Masami Hiramatsu
>
> Software Engineer
> Hitachi Computer Products (America), Inc.
> Software Solutions Division
>
> e-mail: mhiramat(a)redhat.com
>

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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