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- Trace Agent for virtio-trace
- ============================
- Trace agent is a user tool for sending trace data of a guest to a Host in low
- overhead. Trace agent has the following functions:
- - splice a page of ring-buffer to read_pipe without memory copying
- - splice the page from write_pipe to virtio-console without memory copying
- - write trace data to stdout by using -o option
- - controlled by start/stop orders from a Host
- The trace agent operates as follows:
- 1) Initialize all structures.
- 2) Create a read/write thread per CPU. Each thread is bound to a CPU.
- The read/write threads hold it.
- 3) A controller thread does poll() for a start order of a host.
- 4) After the controller of the trace agent receives a start order from a host,
- the controller wake read/write threads.
- 5) The read/write threads start to read trace data from ring-buffers and
- write the data to virtio-serial.
- 6) If the controller receives a stop order from a host, the read/write threads
- stop to read trace data.
- Files
- =====
- README: this file
- Makefile: Makefile of trace agent for virtio-trace
- trace-agent.c: includes main function, sets up for operating trace agent
- trace-agent.h: includes all structures and some macros
- trace-agent-ctl.c: includes controller function for read/write threads
- trace-agent-rw.c: includes read/write threads function
- Setup
- =====
- To use this trace agent for virtio-trace, we need to prepare some virtio-serial
- I/Fs.
- 1) Make FIFO in a host
- virtio-trace uses virtio-serial pipe as trace data paths as to the number
- of CPUs and a control path, so FIFO (named pipe) should be created as follows:
- # mkdir /tmp/virtio-trace/
- # mkfifo /tmp/virtio-trace/trace-path-cpu{0,1,2,...,X}.{in,out}
- # mkfifo /tmp/virtio-trace/agent-ctl-path.{in,out}
- For example, if a guest use three CPUs, the names are
- trace-path-cpu{0,1,2}.{in.out}
- and
- agent-ctl-path.{in,out}.
- 2) Set up of virtio-serial pipe in a host
- Add qemu option to use virtio-serial pipe.
- ##virtio-serial device##
- -device virtio-serial-pci,id=virtio-serial0\
- ##control path##
- -chardev pipe,id=charchannel0,path=/tmp/virtio-trace/agent-ctl-path\
- -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,\
- id=channel0,name=agent-ctl-path\
- ##data path##
- -chardev pipe,id=charchannel1,path=/tmp/virtio-trace/trace-path-cpu0\
- -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel0,\
- id=channel1,name=trace-path-cpu0\
- ...
- If you manage guests with libvirt, add the following tags to domain XML files.
- Then, libvirt passes the same command option to qemu.
- <channel type='pipe'>
- <source path='/tmp/virtio-trace/agent-ctl-path'/>
- <target type='virtio' name='agent-ctl-path'/>
- <address type='virtio-serial' controller='0' bus='0' port='0'/>
- </channel>
- <channel type='pipe'>
- <source path='/tmp/virtio-trace/trace-path-cpu0'/>
- <target type='virtio' name='trace-path-cpu0'/>
- <address type='virtio-serial' controller='0' bus='0' port='1'/>
- </channel>
- ...
- Here, chardev names are restricted to trace-path-cpuX and agent-ctl-path. For
- example, if a guest use three CPUs, chardev names should be trace-path-cpu0,
- trace-path-cpu1, trace-path-cpu2, and agent-ctl-path.
- 3) Boot the guest
- You can find some chardev in /dev/virtio-ports/ in the guest.
- Run
- ===
- 0) Build trace agent in a guest
- $ make
- 1) Enable ftrace in the guest
- <Example>
- # echo 1 > /sys/kernel/debug/tracing/events/sched/enable
- 2) Run trace agent in the guest
- This agent must be operated as root.
- # ./trace-agent
- read/write threads in the agent wait for start order from host. If you add -o
- option, trace data are output via stdout in the guest.
- 3) Open FIFO in a host
- # cat /tmp/virtio-trace/trace-path-cpu0.out
- If a host does not open these, trace data get stuck in buffers of virtio. Then,
- the guest will stop by specification of chardev in QEMU. This blocking mode may
- be solved in the future.
- 4) Start to read trace data by ordering from a host
- A host injects read start order to the guest via virtio-serial.
- # echo 1 > /tmp/virtio-trace/agent-ctl-path.in
- 5) Stop to read trace data by ordering from a host
- A host injects read stop order to the guest via virtio-serial.
- # echo 0 > /tmp/virtio-trace/agent-ctl-path.in
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