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

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Jul 24, 2012 at 12:03 PM, Masami Hiramatsu
<masami.hiramatsu.pt@hitachi.com> wrote:
> (2012/07/24 19:02), Stefan Hajnoczi wrote:
>> On Tue, Jul 24, 2012 at 3:36 AM, Yoshihiro YUNOMAE
>> <yoshihiro.yunomae.ez@hitachi.com> 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
> debugfs.)
>
> 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
> change.

Okay, thanks for explaining some of the ideas you have.  I won't ask
more because it's out of scope for this patch series :).

Stefan