linux/tools/virtio/virtio-trace/README
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   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=charchannel0,\
  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/debug/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
 119