Hello folks! In this post, I am going to talk a bit about QEMU trace-events. I found this mechanism during the development of a virtio driver. Roughly speaking, trace-events are generated by different components of QEMU like the virtio queues, the ioapic, lapics, etc. To enable it, you have to compile QEMU with the following option:
--enable-trace-backends=simple
Then, you have to add the following line to command-line:
-trace events=./events
The file named events contains a list of events that we are interesting to observe. For example, this is mine:
apic_*
ioapic_*
virtio_*
In my case, I am interested in checking if irqs are correctly acknowledged. To see this, I get all the events related with apic, ioapic and virtio. To output the logs in a file, I have to get QEMU monitor and first do 'trace-file on' and second 'trace-file flush'. I am not sure why this is not automatically done. You end up getting a file named 'trace-PID' in which PID is the corresponding PID of the QEMU process. To read this file, you have just to run the following python script:
python3 ~/qemulast/scripts/simpletrace.py ~/qemulast/build/trace-events-all trace-30572
You will get something like:
virtio_mmio_read 131.447 pid=2451 offset=0x60
virtio_mmio_write_offset 141.046 pid=2451 offset=0x64 value=0x1
virtio_mmio_setting_irq 8.345 pid=2451 level=0x0
ioapic_set_irq 4.359 pid=2451 vector=0xc level=0x0
ioapic_eoi_broadcast 29.005 pid=2451 vector=0x2c
ioapic_clear_remote_irr 1.683 pid=2451 n=0xc vector=0x2c
https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/devel/tracing.txt;hb=HEAD.
http://blog.vmsplice.net/2011/02/observability-using-qemu-tracing.html
http://blog.vmsplice.net/2011/03/how-to-write-trace-analysis-scripts-for.html
