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

["Daniel P. Berrange" <berrange@redhat.com>]

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.


Regards,
Daniel
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