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.

Stefan