freebsd-head: suddenly NMI panics lead to ddb being unable to stop CPUs?

[Ian Lepore]

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).

On ARM we have a really nice wait-for-event/signal-event pair of
instructions, where WFE would be a natural for cpu_spinwait(), but would
require a matching cpu_spinwake() type call to be sprinkled around to do
the matching SEV instruction.

-- Ian