Re: proposed instruction trace support in SystemTap

[Dave Nomura <dcnltc@us.ibm.com>]

I've been looking into the kernel API for handling single stepping and 
haven't really found anything.  ptrace() is used by gdb but it's usage 
model might be overly restrictive for what we want:  we would have to 
have a parent process then uses ptrace() to trace it's children.  
ptrace() also does not trace into the kernel which is an ITRACE requirement.

I think the requirement of tracing into the kernel is only needed in 
some scenarios and may only be needed for the ITRACE application of 
SystemTap instruction tracing.  In its most general form Perfomance 
Inspector ITRACE allows tracing into the kernel and tracing of a whole 
range of processes.  Due to these requirements it places usage 
constraints on the user that requires exclusive access ot a machine.  In 
the more common SystemTap instruction tracing scenario only a single 
process is being traced, and only user code is traced maybe it would be 
approriate to use ptrace() to do the single stepping.

It has already been suggested that we have different APIs for ITRACE vs. 
simpler (non-kernel tracing) instruction tracing modes so it might be a 
simple matter of telling  the SystemTap translator what kind of trap 
handler to generate (or referencd from the runtime stap scripts).  In 
the non-kernel-tracing-single-process scenario just the normal process 
switch management of registers will handle restoring the single step 
trap bit, or if ptrace() (or possibly utrace()).

The ITRACE-kernel-tracing scenario might require similar usage 
restrictions as PI ITRACE, and we simply would require that you aren't 
using other kernel debuggers (xmon, kgdb,...) while trying to do an 
ITRACE to avoid conflict over the kernel resources needed for 
instruction tracing.   Alternatively, a kernel API (if it doesn't 
already exist) for handling these kernel resources could be created.  I 
have heard that xmon and kgdb both use the __debugger_sstep() trap 
handler pointer.
Frank Ch. Eigler wrote:
>
>> [...] There is basically one single instruction trap handler that
>> the stap translator will generate with logic to figure out what
>> handler code to run [...]
>>     
>
> The "existing kernel API" is the key issue here.  How exactly does one
> activate single-stepping traps on each of the interesting
> architectures, and on multiple different kernel generations (RHEL)?
> How does one hook into the handling system correctly (avoiding
> interference to other consumers of trap data like gdb, uprobes)?
>
> These questions need answers in order for systemtap to generate code
> to implement this.
>
>
>   
> - FChE
>
>   


-- 
Dave Nomura
LTC Linux Power Toolchain