[Bug 1916112] Re: Illegal instruction crash of QEMU on Jetson Nano

[Peter Maydell]

Can you use gdb to display what the instruction that provoked the SIGILL
is ? ("disas $pc-32,$pc+32" or similar should do it).

Re "Why is TCG used as the accelerator when KVM is present?", the answer
is that only certain board types and CPU types work with KVM. The simple
answer is "only the 'virt' board works with KVM". Other boards generally
use a CPU type or CPU features which KVM does not support and so TCG is
the only choice. It's a QEMU bug that we assert() rather than printing a
helpful error message (which we will probably fix for 6.0.)

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Title:
  Illegal instruction crash of QEMU on Jetson Nano

Status in QEMU:
  New

Bug description:
  I have a jetson nano (arm64 SBC) and I want to check the native
  emulation performance of Raspbian Buster. I used the info available
  here:

  https://github.com/dhruvvyas90/qemu-rpi-kernel/tree/master/native-
  emuation

  I have Xubuntut 20.04 with KVM enabled kernel running on the Jetson
  Nano

  However QEMU crashes with "Illegal Instruction" during kernel boot. I
  have a built latest QEMU from sources with following configuration

  ./configure --prefix=/usr/local --target-list=aarch64-softmmu,arm-
  softmmu  --enable-guest-agent --enable-vnc  --enable-vnc-jpeg
  --enable-vnc-png --enable-kvm --enable-spice --enable-sdl --enable-gtk
  --enable-virglrenderer --enable-opengl

  qemu-system-aarch64 --version
  QEMU emulator version 5.2.50 (v5.2.0-1731-g5b19cb63d9)

  When I run as follows:

  ../build/qemu-system-aarch64 -M raspi3
  -append "rw earlyprintk loglevel=8 console=ttyAMA0,115200 
dwc_otg.lpm_enable=0 root=/dev/mmcblk0p2 rootdelay=1"
  -dtb ./bcm2710-rpi-3-b-plus.dtb
  -sd 
/media/96747D21747D0571/JetsonNano/2020-08-20-raspios-buster-armhf-full.qcow2
  -kernel ./kernel8.img
  -m 1G -smp 4 -serial stdio -usb -device usb-mouse -device usb-kbd

  I get :
  [ 74.994834] systemd[1]: Condition check resulted in FUSE Control File System 
being skipped.
  [ 76.281274] systemd[1]: Starting Apply Kernel Variables...
  Starting Apply Kernel Variables...
  Illegal instruction (core dumped)

  When I use GDB I see this:

  Thread 8 "qemu-system-aar" received signal SIGILL, Illegal instruction.
  [Switching to Thread 0x7fad7f9ba0 (LWP 28037)]
  0x0000007f888ac690 in code_gen_buffer ()
  (gdb) bt
  #0 0x0000007f888ac690 in code_gen_buffer ()
  #1 0x0000005555d7c038 in cpu_tb_exec (tb_exit=, itb=, cpu=0x7fb4502c40)
  at ../accel/tcg/cpu-exec.c:191
  #2 cpu_loop_exec_tb (tb_exit=, last_tb=, tb=, cpu=0x7fb4502c40)
  at ../accel/tcg/cpu-exec.c:708
  #3 cpu_exec (cpu=cpu@entry=0x7fb4502c40) at ../accel/tcg/cpu-exec.c:819
  ..

  I have just two questions:

  Is this a problem with QEMU or is there anything specific build or
  options I need to use. Any specific version of QEMU should be used ?

  Why is TCG used as the accelerator when KVM is present. Is it possible
  and how to use KVM ?

  If I enabled the KVM then I get this error:

  ../build/qemu-system-aarch64 -M raspi3 -enable-kvm -append "rw earlyprintk 
loglevel=8 console=ttyAMA0,115200 dwc_otg.lpm_enable=0 root=/dev/mmcblk0p2 
rootdelay=1" -dtb ./bcm2710-rpi-3-b-plus.dtb -sd 
/media/96747D21747D0571/JetsonNano/2020-08-20-raspios-buster-armhf-full.qcow2 
-kernel ./kernel8.img -m 1G -smp 4 -serial stdio -usb -device usb-mouse -device 
usb-kbd
  WARNING: Image format was not specified for 
'/media/96747D21747D0571/JetsonNano/2020-08-20-raspios-buster-armhf-full.img' 
and probing guessed raw.
           Automatically detecting the format is dangerous for raw images, 
write operations on block 0 will be restricted.
           Specify the 'raw' format explicitly to remove the restrictions.
  qemu-system-aarch64: ../softmmu/physmem.c:750: cpu_address_space_init: 
Assertion `asidx == 0 || !kvm_enabled()' failed.

  Thanks a lot.

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