kern/55352: access_regs_set_unaligned_pc_0x7 test cases sometimes fail

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         55352
>Category:       kern
>Synopsis:       access_regs_set_unaligned_pc_0x7 test cases sometimes fail
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jun 06 11:20:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

The access_regs_set_unaligned_pc_0x7 test case in multiple
lib/libc/sys/t_ptrace_wait* tests is now randomly failing on my amd64
bare metal testbed which runs the tests on a physical HP DL360 G7
system.  Each test case fails in maybe 3/4 of the runs, but there's
six of them, so at least one of them is almost certain to fail in
each run.

Here is a tabular representation of the outcomes of the last 30 test
runs, with passing test cases marked by "-" and failing ones by "X":

  -------------------------XX-X-X-X-X--XXX   lib/libc/sys/t_ptrace_wait3:access_regs_set_unaligned_pc_0x7
  -------------------------X--XXXXX-X-XXXX   lib/libc/sys/t_ptrace_wait4:access_regs_set_unaligned_pc_0x7
  -------------------------X-XX--XXX-X-X-X   lib/libc/sys/t_ptrace_wait6:access_regs_set_unaligned_pc_0x7
  --------------------------XXX-X--XXX-XX-   lib/libc/sys/t_ptrace_wait:access_regs_set_unaligned_pc_0x7
  -------------------------XX-XXXXXX-XXXX-   lib/libc/sys/t_ptrace_waitid:access_regs_set_unaligned_pc_0x7
  -------------------------XXX--XXXXXX---X   lib/libc/sys/t_ptrace_waitpid:access_regs_set_unaligned_pc_0x7

Although the failures are easily reproduced on the system in case,
I'm having a hard time reproducing it on other systems.  None of the
following are failing:

 - Tetsts run in qemu
 - Tests run on a Dell R630 rather than a HP DL360
 - Tests run on the same hardware under NetBSD/i386 rather than NetBSD/amd64
 - The access_regs_set_unaligned_pc_0xN test case for any N other than 7

The first failures appeared after this commit:

  2020.05.31.01.39.33 ad src/sys/dev/acpi/acpi_cpu_cstate.c 1.61

Logs:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/commits-2020.05.html#2020.05.31.01.39.33

>How-To-Repeat:

With difficulty.

>Fix: