Insta-reset-bootloop with r328166 and vm.pmap.pcid_enabled=1

[Konstantin Belousov]

On Thu, Jan 25, 2018 at 04:58:04PM -0500, Ian FREISLICH wrote:
> Hi
> 
> I cannot for the life of me recall why I had vm.pmap.pcid_enabled=1 set
> in loader.conf, but I did.  Most likely very historical reasons.
> 
> r328166 and later reset without dropping into the debugger during boot,
> no panic message.

Yes, this is how it is currently behaves.
PCID can be used to optimize PTI, see https://reviews.freebsd.org/D13985.
It is used for very differrent algorithm when PTI=1, comparing with PTI=0.