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

[Daniel P. Berrange]

On Mon, Sep 05, 2016 at 08:29:54PM +0200, Lluís Vilanova wrote:
> Daniel P Berrange writes:
> 
> > On Mon, Aug 29, 2016 at 08:46:02PM +0200, Lluís Vilanova wrote:
> >> Stefan Hajnoczi writes:
> >> 
> >> > When SystemTap is used the QEMU monitor interface does nothing.
> >> 
> >> That's not what I've experienced. I was able to use a stap script to change the
> >> tracing state of events:
> >> 
> >> #!/usr/bin/env stap
> >> 
> >> %{
> >> #include </home/lluis/Projects/qemu-dbi-test/test.h>
> >> %}
> >> 
> >> function event:long(cpu:long, addr:long, info:long)
> >> %{
> >> char *argv[4] = {"/bin/sh", "-c", "echo 'trace-event * off' | telnet localhost 1234", NULL};
> >> call_usermodehelper(argv[0], argv, NULL, UMH_WAIT_EXEC);
> >> STAP_RETURN(0);
> >> %}
> 
> > I don't know what you're trying to achieve here.  The trace-event state,
> > as changed/viewed via QEMU monitor, is irrelevant to the dtrace (systemtap)
> > backend. dtrace and ltt-ust are both fully dynamic trace event backends,
> > so the QEMU event state has no effect on them. The probe points in the
> > binary are dynamically enabled / disabled by the dtrace runtime. ie dtrace
> > will automatically enable an event if you write a dtrace script that uses
> > the event.
> 
> Sorry, I did not properly explain the use case. This is an example of using
> QEMU's tracing infrastructure to control itself. Here I'm using the "log"
> backend to trace events to disk, and the "dtrace" backend (systemtap) to control
> the tracing state of such events.

Ah, I see. I guess I'd personally just have systemtap/dtrace directly emit
the data to be recorded, and avoid the log backend entirely.

Regards,
Daniel
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