[Bug 243225] "mpr0: Out of chain frames" boot hang after clang 9.0.1 import (probably timing, not compiler related)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=243225

--- Comment #6 from Terry Kennedy <terry-freebsd at glaver.org> ---
(In reply to Terry Kennedy from comment #5)
Following up to my own post.

I just emailed Warner Losh the following (forgetting that the discussion was
also going on in this PR):

  The system is a Dell PowerEdge R730 with dual E5-2643 v3 CPUs, 128GB
RAM, PERC H730 Mini RAID controller (for internal SAS drives) and a
Dell "12Gb External SAS Adapter" (LSI SAS 3008) which is the problem
controller. Since I emailed you last, Dell changed it again (they're
getting irritated at this point) with no change.

  With a r364046 kernel from today, the system boots fine with hyper-
threading enabled, but shows thenmprmhang with hyperthreading disabled.
The previous kernel worked with hyperthreading disabled - as I've said,
this appears to be timing-related as it first showed up after the LLVM
9.0.0 import into 12-STABLE.

  A verbose boot didn't appear to show anything, but as the kernel mes-
sage buffer never gets written to disk because of the hang, I have to
play the boot sequence back as a video from the internal remote manage-
ment card. Also, sometimes a verbose boot works, since this is a timing-
related Heisenbug, apparently.

  I have created a directory https://www.glaver.org/transient/imp which
has (so far) dmesg.boot and pciconf.txt from a good boot, bootcapture_
bad.dvc from a non-hyperthreaded boot hang, bootcapture_good.dvc from
a successful hyperthreaded boot. and videoplayer.zip which is the play-
er for .dvc files. It contains a Windows .exe and a jar file and shell
script for Linux (and presumably FreeBSD with Java and a GUI).

  I can provide remote access to the Dell iDRAC 8 Enterprise card which
allows console viewing and all sorts of other stuff, like reset / power
cycle and access to BIOS setup, etc. if needed.

  Let me know if there is anything else you need.

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