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

[Lluís Vilanova]

Daniel P Berrange writes:

> 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.

Backends with a buffered output stream like "simple" should be faster than
"dtrace", which AFAIK injects a software trap. Also, you can take advantage of
existing QEMU infrastructure to control the tracing of events, instead of
implementing your own on each script.

Cheers,
  Lluis