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Date: Fri, 21 Aug 2015 20:10:48 +0300
From: Konstantin Belousov <kostikbel@gmail.com>
To: Julian Elischer <julian@freebsd.org>
Cc: Ian Lepore <ian@freebsd.org>, freebsd-current <freebsd-current@freebsd.org>
Subject: Re: freebsd-head: suddenly NMI panics lead to ddb being unable to
 stop CPUs?
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On Sat, Aug 22, 2015 at 12:53:15AM +0800, Julian Elischer wrote:
> On 8/22/15 12:23 AM, Ian Lepore wrote:
> > On Fri, 2015-08-21 at 23:30 +0800, Julian Elischer wrote:
> >> On 8/21/15 11:25 PM, Adrian Chadd wrote:
> >>> Ah, cool. I'll give it a whirl.
> >>>
> >>> I'm a little worried about having all of the other cores spinning in
> >>> this case (mostly thermal; the machines get VERY LOUD when the CPUs
> >>> are spinning..)
> >>>
> >> make each spin with the pause instruction.. and for N seconds (N being
> >> the CPU ID) or something
> > cpu_spinwait() is defined as ia32_pause() on i386 and amd64.
> >
> > The problem is that ONLY x86 arches benefit from cpu_spinwait(), it's
> > just a nop on other arches, which turn into little radiant heaters for
> > long spins (like while waiting for APs to be released).
> 
> remind me again why it cant spin with a 'halt' instruction?
Because interrupts are disabled.  Even NMI is disabled, because spin
happens in the NMI handler.  There is no way to wake up CPU from the
halted state.