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Re: [Qemu-devel] [RFC PATCH 0/6] virtio-trace: Support virtio-trace

From: Yoshihiro YUNOMAE
Subject: Re: [Qemu-devel] [RFC PATCH 0/6] virtio-trace: Support virtio-trace
Date: Tue, 24 Jul 2012 20:19:51 +0900
User-agent: Mozilla/5.0 (Windows NT 5.2; rv:13.0) Gecko/20120604 Thunderbird/13.0

Hi Stefan,

Thank you for commenting on our patch set.

(2012/07/24 20:03), Masami Hiramatsu wrote:
(2012/07/24 19:02), Stefan Hajnoczi wrote:
On Tue, Jul 24, 2012 at 3:36 AM, Yoshihiro YUNOMAE
<address@hidden> wrote:
The performance of each method is compared as follows:
  [1] Native
      - only recording trace data to ring-buffer on a guest
  [2] Virtio-trace
      - running a trace agent on a guest
      - a reader on a host opens FIFO using cat command
  [3] IVRing
      - A SystemTap script in a guest records trace data to IVRing.
        -- probe points are same as ftrace.
  [4] Virtio-serial(normal)
      - A reader(using cat) on a guest output trace data to a host using
        standard output via virtio-serial.

The first time I read this I thought you are adding a new virtio-trace
device.  But it looks like this series really add splice support to
virtio-console and that yields a big performance improvement when
sending trace_pipe_raw.

Yes, sorry for the confusion. Actually this is an enhancement of
virtio-serial. I'm working with Yoshihiro on this feature.

Guest ftrace is useful and I like this.  Have you thought about
controlling ftrace from the host?  Perhaps a command could be added to
the QEMU guest agent which basically invokes trace-cmd/perf.

As you can see, guest trace-agent can be controlled via a
control channel. In our scenario, host tools can control that
instead of guest one.

We are considering that exporting the tracing part of guest's
debugfs to host via another virtio-serial channel by using
9pfs, so that the host tools can refer that.

(In this scenario, guest trace-agent will also provide 9pfs server.
Since it means that the agent can handle writing a special file,
trace-agent can be controlled via the special file on exported

Of course, this also requires modifying trace-cmd/perf to accept
some options like guest-debugfs mount point, guest's serial
channel pipe (or unix socket?), etc. However, it will be a small

Thank you,

>> Are you using text formatted ftrace?
No, currently using raw format, but we'd like to reformat it in text.

Thank you,

Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: address@hidden

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