Re: [Qemu-devel] [PATCH 0/6] hypertrace: Lightweight guest-to-QEMU trace channel

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Aug 24, 2016 at 12:25:54PM +0200, Lluís Vilanova wrote:
> Stefan Hajnoczi writes:
> 
> > On Sun, Aug 21, 2016 at 02:32:34PM +0200, Lluís Vilanova wrote:
> >> Unfortuntely, I've been unable to to make dtrace recognise QEMU's events (I'm
> >> only able to see the host kernel events). If someone with more experience on it
> >> can help me use dtrace with QEMU's events, I'll also add the supporting library
> >> to let dtrace do the callout to QEMU's moitor interface and control the events,
> >> and add a prperly useful example of that on the hypertrace docs (which was my
> >> original intention).
> 
> > Which "dtrace" and host OS are you using?
> 
> > QEMU builds with static user-space probes.  You need to tell DTrace or
> > SystemTap to enable those probes in order to record trace data.
> 
> I'm using debian on a 4.6.0-1-amd64 kernel with systemtap 3.0.6.
> 
> I just gave it another try, and works if I use probes like:
> 
>   process("<binary-path>").mark("<event-name>")
> 
> although they don't seem to appear on "stap -l" or anything like that (I cannot
> find a "qemu" provider). But I'm still unable to print the event values. This:

The following enumerates events for me:

$ sudo stap -L 'process("/usr/bin/qemu-system-x86_64").mark("*")'
process("/usr/bin/qemu-system-x86_64").mark("alsa_no_frames") $arg1:long
process("/usr/bin/qemu-system-x86_64").mark("alsa_pollout") $arg1:long $arg2:long
process("/usr/bin/qemu-system-x86_64").mark("alsa_read_zero") $arg1:long
...

You can also use /usr/share/systemtap/tapset/qemu-system-x86_64.stp.
I'm running Fedora 24 with qemu-system-x86 2.6.0-5.fc24 and SystemTap
3.0-3.fc24.

> Also, I'm still not sure how to interact with QEMU's monitor interface from
> within the probe code (probes execute in kernel mode, including "guru mode"
> code).

When SystemTap is used the QEMU monitor interface does nothing.

Stefan

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