Internal error: Oops - BUG() / kvm boot race - arm64 kpti patchset related

[Paolo Pisati]

Reproducible on 4.16-rc3 and 4.4.20 using defconfig - the failure it's
intermittent, but i could reproduce it 100% if i boot loop the kvm instance 
(it usually shows up in less than 10 iterations but i tested 32 boots before
marking it good).

I bisected it down to this interval in linux-4.14.y:

2feb36e arm64: kpti: Add ->enable callback to remap swapper using nG mappings
ee28fed arm64: mm: Permit transitioning from Global to Non-Global without BBM
6928820 arm64: kpti: Make use of nG dependent on arm64_kernel_unmapped_at_el0()
c98c8c2 arm64: Turn on KPTI only on CPUs that need it

c98c8c2 is good, 2feb36e is bad - couldn't bisect in between, since it didn't
boot there.
And yes, when i tested 2feb36e i applied the "el1 trashing fix" mentioned here:
https://www.spinics.net/lists/arm-kernel/msg636489.html

Anything else i can do to help debug this?
-- 
bye,
p.