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Early Kernel Boot Debug

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Manohar Castelino edited this page Oct 22, 2018 · 1 revision

How to debug early boot kernel crashes

On the virt platform if the kernel crashes early (even before the virtio-serial port comes up), you need to be able to debug the early boot process. This can be done using the sysbug-debugcon device which is available right at boot.

The key items to note below are

  • Kernel commandline: console=ttyS0
  • Additional sysbus-debugcon device: -device sysbus-debugcon,iobase=0x3f8,chardev=serialcon -chardev file,path=/tmp/serial-log,id=serialcon

Note:

  • 0x3f8 maps to COM1. You can have additional serial ports if needed.
  • As modeled by sysbus-debugcon these are output only ports
dos     common                  IO    
name     name     major minor address 
COM1   /dev/ttyS0  4,  64;   3F8      
COM2   /dev/ttyS1  4,  65;   2F8      
COM3   /dev/ttyS2  4,  66;   3E8      
COM4   /dev/ttyS3  4,  67;   2E8      

     QEMU=~/build-x86_64/x86_64-softmmu/qemu-system-x86_64
     KERNEL=~/linux/arch/x86_64/boot/bzImage
     BIOS=~/ovmf.fd.virt
     OSIMAGE=~/clear.img

sudo $QEMU \
     -kernel $KERNEL -append 'console=ttyS0 console=hvc0 root=/dev/vda3 rw rootfstype=ext4 data=ordered rcupdate.rcu_expedited=1 tsc=reliable no_timer_check noapictimer log_buf_len=1M debug loglevel=7' \
     -bios $BIOS \
     -nographic \
     -nodefaults \
     -L . \
     -net none \
     -machine virt,accel=kvm,kernel_irqchip \
     -smp 4,maxcpus=8\
     -m 512M,slots=3,maxmem=4G \
     -device virtio-serial-pci,id=virtio-serial0 -device virtconsole,chardev=charconsole0,id=console0 -chardev stdio,id=charconsole0 \
     -device virtio-blk-pci,drive=image -drive if=none,id=image,file=$OSIMAGE,format=raw \
     -monitor telnet:127.0.0.1:55555,server,nowait \
     -netdev user,id=mynet0,hostfwd=tcp::${VMN}0022-:22 -device virtio-net-pci,netdev=mynet0 \
     -device sysbus-debugcon,iobase=0x402,chardev=debugcon -chardev file,path=/tmp/debug-log,id=debugcon \
     -device sysbus-debugcon,iobase=0x3f8,chardev=serialcon -chardev file,path=/tmp/serial-log,id=serialcon

You can now observe the kernel boot prints at /tmp/serial-log and the firmware bootup prints at /tmp/debug-log

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