With a client we're running into the following boot panic
since upgrading from 5.7 to 5.9 on a specific Hyper-V guest:

cpu0: Intel(R) Xeon(R) CPU E5420 @ 2.50GHz ("GenuineIntel" 686-class) 1.65 GHz
kernel: protection fault trap, code=0
Stopped at cpu_paenable+0x20: movl %edi,%cr3

This happens consistently, but we have other clients that
run Hyper-V without a problem on 5.9 now.

Looking for a clue I dug through the commits and found that
reviving pmap_bootstrap_pae() in April 2015 caused this, and
reverting boots said Hyper-V guest just fine.

Hyper-V is on the latest version and we could not find a
reason why said guest / host pair behaves that way.

I have two questions:

(1) Since NX/PAE is pretty much standard on all hardware from
the last +15 years and is now used to enforce W^X, are there
going to be problems *not* running that code either now or in
the future?

(2) Could this be something to be fixed in the i386 assembler
code in cpu_paenable?

It's difficult to get dumps / screen caps from the system or
run commands in ddb, but if there's interest I will try to
provide the requested info.


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