QEMU simple backend tracing dosen't print anything - linux

Question

I'm doing get simple trace file from QEMU.
I followed instructions docs/tracing.txt
with this command "qemu-system-x86_64 -m 2G -trace events=/tmp/events ../qemu/test.img"
i'd like to get just simple trace file.
i've got trace-pid file, however, it dosen't have anything in it.
Build with the 'simple' trace backend:
./configure --enable-trace-backends=simple
make
Create a file with the events you want to trace:
echo bdrv_aio_readv > /tmp/events
echo bdrv_aio_writev >> /tmp/events
Run the virtual machine to produce a trace file:
qemu -trace events=/tmp/events ... # your normal QEMU invocation
Pretty-print the binary trace file:
./scripts/simpletrace.py trace-events trace-* # Override * with QEMU
i followd this instructions.
please somebody give me some advise for this situation.
THANKS!

Answer 1

I got same problem by following the same document.
https://fossies.org/linux/qemu/docs/tracing.txt
got nothing because
bdrv_aio_readv and bdrv_aio_writev was not enabled by default, at least the version I complied, was not enabled. you need to open trace-events under source directory, looking for some line without disabled, e.g. I using:
echo "load_file" > /tmp/events
Then start qemu,
after a guest started, I run
./scripts/simpletrace.py trace-events trace-Pid
I got
load_file 1474.156 pid=5249 name=kvmvapic.bin path=qemu-2.8.0-rc0/pc-bios/kvmvapic.bin
load_file 22437.571 pid=5249 name=vgabios-stdvga.bin path=qemu-2.8.0-rc0/pc-bios/vgabios-stdvga.bin
load_file 10034.465 pid=5249 name=efi-e1000.rom
you can also add -monitor stdio to qemu command line, after it started, you can the following command in qemu CLI:
(qemu) info trace-events
load_file : state 1
vm_state_notify : state 1
balloon_event : state 0
cpu_out : state 0
cpu_in : state 0
1 means enabled events.

Answer 2

Modify the trace-events file in the source tree
As of v2.9.0 you also have to remove the disable from the lines you want to enable there, e.g.:
-disable exec_tb(void *tb, uintptr_t pc) "tb:%p pc=0x%"PRIxPTR
+exec_tb(void *tb, uintptr_t pc) "tb:%p pc=0x%"PRIxPTR
and recompile.
Here is a minimal fully automated runnable example that boots Linux and produces traces: https://github.com/cirosantilli/linux-kernel-module-cheat
For example, I used the traces to count how many boot instructions Linux has: https://github.com/cirosantilli/linux-kernel-module-cheat/blob/c7bbc6029af7f4fab0a23a380d1607df0b2a3701/count-boot-instructions.md
I have a lightly patched QEMU as a submodule, the key commit is: https://github.com/cirosantilli/qemu/commit/e583d175e4cdfb12b4812a259e45c679743b32ad