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

[Masami Hiramatsu]

On Mon, 05 Sep 2016 16:37:01 +0200
Lluís Vilanova <address@hidden> wrote:

> Stefan Hajnoczi writes:
> 
> > On Mon, Aug 29, 2016 at 08:46:02PM +0200, Lluís Vilanova wrote:
> >> >> 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.
> >> 
> >> 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);
> >> %}
> >> 
> >> probe begin {
> >> printf("hello\n")
> >> }
> >> probe 
> >> process("./install/vanilla/bin/qemu-system-i386").mark("guest_mem_before_exec")
> >> {
> >> printf("%x %d %d\n", $arg1, $arg2, $arg3)
> >> event($arg1, $arg2, $arg3)
> >> exit()
> >> }
> >> 
> >> The only caveat is that you must pass the "-g" argument to stap.
> >> 
> >> Also, for some reason the printf in the probe always prints zeros, no 
> >> matter
> >> what the actual event receives (I've debugged QEMU down to the call to the
> >> auto-generated stap functions). Could this be an error in systemtap?
> 
> > It's strange that arguments do not have valid values.  Debugging the
> > stap functions is the next step if you want to figure out what happened.
> > I've never had this issue before so maybe something with Debian
> > SystemTap userspace probes is broken.
> 
> I already debugged it, to the point where QEMU executes the trap injected by
> systemtap, and the register values that were supposed to hold the arguments 
> are
> correct.
> 
> I suppose that if you execute the stap script I pasted it will show the proper
> values. Then it's definitely a problem with Debian's userspace probes.

Would you have tried to update your kernel to mainline and tested it ?
If it occurs, you also should try to use a raw uprobe via ftrace(uprobe_events)
and perftools.
If you have the latest perf (maybe you'll need checkout the latest tip tree),
you can use SDT as below (currently it doesn't support args, so you'll need
debuginfo.)

# perf buildid-cache --add ./install/vanilla/bin/qemu-system-i386
# perf probe -x ./install/vanilla/bin/qemu-system-i386 -a 
'guest_mem_before_exec $vars'

And you'll see new event is registered which can be traced by ftrace or perf.

Thanks,

-- 
Masami Hiramatsu <address@hidden>