README

  1Trace Agent for virtio-trace
  2============================
  3
  4Trace agent is a user tool for sending trace data of a guest to a Host in low
  5overhead. Trace agent has the following functions:
  6 - splice a page of ring-buffer to read_pipe without memory copying
  7 - splice the page from write_pipe to virtio-console without memory copying
  8 - write trace data to stdout by using -o option
  9 - controlled by start/stop orders from a Host
 10
 11The trace agent operates as follows:
 12 1) Initialize all structures.
 13 2) Create a read/write thread per CPU. Each thread is bound to a CPU.
 14    The read/write threads hold it.
 15 3) A controller thread does poll() for a start order of a host.
 16 4) After the controller of the trace agent receives a start order from a host,
 17    the controller wake read/write threads.
 18 5) The read/write threads start to read trace data from ring-buffers and
 19    write the data to virtio-serial.
 20 6) If the controller receives a stop order from a host, the read/write threads
 21    stop to read trace data.
 22
 23
 24Files
 25=====
 26
 27README: this file
 28Makefile: Makefile of trace agent for virtio-trace
 29trace-agent.c: includes main function, sets up for operating trace agent
 30trace-agent.h: includes all structures and some macros
 31trace-agent-ctl.c: includes controller function for read/write threads
 32trace-agent-rw.c: includes read/write threads function
 33
 34
 35Setup
 36=====
 37
 38To use this trace agent for virtio-trace, we need to prepare some virtio-serial
 39I/Fs.
 40
 411) Make FIFO in a host
 42 virtio-trace uses virtio-serial pipe as trace data paths as to the number
 43of CPUs and a control path, so FIFO (named pipe) should be created as follows:
 44	# mkdir /tmp/virtio-trace/
 45	# mkfifo /tmp/virtio-trace/trace-path-cpu{0,1,2,...,X}.{in,out}
 46	# mkfifo /tmp/virtio-trace/agent-ctl-path.{in,out}
 47
 48For example, if a guest use three CPUs, the names are
 49	trace-path-cpu{0,1,2}.{in.out}
 50and
 51	agent-ctl-path.{in,out}.
 52
 532) Set up of virtio-serial pipe in a host
 54 Add qemu option to use virtio-serial pipe.
 55
 56 ##virtio-serial device##
 57     -device virtio-serial-pci,id=virtio-serial0\
 58 ##control path##
 59     -chardev pipe,id=charchannel0,path=/tmp/virtio-trace/agent-ctl-path\
 60     -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,\
 61      id=channel0,name=agent-ctl-path\
 62 ##data path##
 63     -chardev pipe,id=charchannel1,path=/tmp/virtio-trace/trace-path-cpu0\
 64     -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,\
 65      id=channel1,name=trace-path-cpu0\
 66      ...
 67
 68If you manage guests with libvirt, add the following tags to domain XML files.
 69Then, libvirt passes the same command option to qemu.
 70
 71	<channel type='pipe'>
 72	   <source path='/tmp/virtio-trace/agent-ctl-path'/>
 73	   <target type='virtio' name='agent-ctl-path'/>
 74	   <address type='virtio-serial' controller='0' bus='0' port='0'/>
 75	</channel>
 76	<channel type='pipe'>
 77	   <source path='/tmp/virtio-trace/trace-path-cpu0'/>
 78	   <target type='virtio' name='trace-path-cpu0'/>
 79	   <address type='virtio-serial' controller='0' bus='0' port='1'/>
 80	</channel>
 81	...
 82Here, chardev names are restricted to trace-path-cpuX and agent-ctl-path. For
 83example, if a guest use three CPUs, chardev names should be trace-path-cpu0,
 84trace-path-cpu1, trace-path-cpu2, and agent-ctl-path.
 85
 863) Boot the guest
 87 You can find some chardev in /dev/virtio-ports/ in the guest.
 88
 89
 90Run
 91===
 92
 930) Build trace agent in a guest
 94	$ make
 95
 961) Enable ftrace in the guest
 97 <Example>
 98	# echo 1 > /sys/kernel/tracing/events/sched/enable
 99
1002) Run trace agent in the guest
101 This agent must be operated as root.
102	# ./trace-agent
103read/write threads in the agent wait for start order from host. If you add -o
104option, trace data are output via stdout in the guest.
105
1063) Open FIFO in a host
107	# cat /tmp/virtio-trace/trace-path-cpu0.out
108If a host does not open these, trace data get stuck in buffers of virtio. Then,
109the guest will stop by specification of chardev in QEMU. This blocking mode may
110be solved in the future.
111
1124) Start to read trace data by ordering from a host
113 A host injects read start order to the guest via virtio-serial.
114	# echo 1 > /tmp/virtio-trace/agent-ctl-path.in
115
1165) Stop to read trace data by ordering from a host
117 A host injects read stop order to the guest via virtio-serial.
118	# echo 0 > /tmp/virtio-trace/agent-ctl-path.in