KVM_GUEST support breaks page fault tracing

[Dave Hansen]

I noticed on some of my systems that page fault tracing doesn't work:

	cd /sys/kernel/debug/tracing
	echo 1 > events/exceptions/enable
	cat trace;
	# nothing shows up

I eventually traced it down to CONFIG_KVM_GUEST.  At least in a KVM VM,
enabling that option breaks page fault tracing, and disabling fixes it.
 I tried on some old kernels and this does not appear to be a
regression: it never worked.

Anybody have any theories about what is going on?

I'm also having problems with the page fault tracing on bare metal (same
symptom of no trace output).  I'm unsure if it's related.
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