kern/56524: NVMM: Unexpected VM exit code 0xffffffffffffffff [hw=0x31]

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

>Number:         56524
>Category:       kern
>Synopsis:       NVMM: Unexpected VM exit code 0xffffffffffffffff [hw=0x31]
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Nov 26 19:25:00 +0000 2021
>Originator:     Andreas Gustafsson
>Release:        NetBSD 9.2
>Organization:
>Environment:
System: NetBSD guido.araneus.fi
Architecture: x86_64
Machine: amd64
>Description:

I had been successfully running a FreeBSD guest on a NetBSD host using
qemu -accel nvmm for several days, but today I found that the qemu
process had dumped core:

  qemu-system-x86_64: NVMM: Unexpected VM exit code 0xffffffffffffffff [hw=0x31]
  qemu-system-x86_64: NVMM: Failed to execute a VCPU.
  [1]   Abort trap (core dumped) qemu-system-x86_64 -hda freebsd12a.qcow -m 204...

The qemu command line was:

  qemu-system-x86_64 -hda freebsd12a.qcow -m 2048 -accel nvmm -smp 8 -nographic -netdev tap,id=eth,ifname=tap0,script=no,downscript=no -device e1000,netdev=eth,mac=52:54:00:00:00:01

and the core file shows this backtrace:

  (gdb) bt
  #0  0x000078d330b679aa in _lwp_kill () from /usr/lib/libc.so.12
  #1  0x000078d330b6725a in abort () at /usr/src/lib/libc/stdlib/abort.c:74
  #2  0x00000001b5bcbb19 in nvmm_vcpu_exec (cpu=cpu@entry=0x78d3369af240) at ../target/i386/nvmm/nvmm-all.c:1022
  #3  0x00000001b5bd7c21 in qemu_nvmm_cpu_thread_fn (arg=arg@entry=0x78d3369af240) at ../target/i386/nvmm/nvmm-accel-ops.c:45
  #4  0x00000001b5eabd02 in qemu_thread_start (args=<optimized out>) at ../util/qemu-thread-posix.c:541
  #5  0x000078d33100c072 in pthread__create_tramp (cookie=0x78d3369c6000) at /usr/src/lib/libpthread/pthread.c:592
  #6  0x000078d330a87bb0 in ?? () from /usr/lib/libc.so.12
  #7  0x0000000000200000 in ?? ()
  #8  0x0000000000000000 in ?? ()

>How-To-Repeat:

>Fix: