Re: kernel API for in-kernel single stepping

[Paul Mackerras <paulus at au1 dot ibm dot com>]

Dave Nomura writes:

>    It looks like you have vast experience with single stepping in the 
> kernel (sstep.[hc]).  In a previous mail you mentioned that there is 
> only a "scouts honor" convention with use of the kernel's single step 
> trapping mechanism that is followed by the kernel debuggers with only 
> one client at a time using the resources.   With the various kernel 
> debuggers, kprobes, itrace, and maybe others trying to share these 
> resources do you think it is time to develop some sort of kernel single 
> stepping API?   Frank is requesting this  API  before  changing  
> SystemTap to support  single step traps.

At present kprobes sets the MSR_SE bit in the MSR when it wants to
single-step, and uses the notify_die infrastructure as the way it gets
notified when the single-step trap occurs.  It would be reasonable for
other things that want to use single-stepping to do the same, i.e. set
MSR_SE when they want to single-step, and use the notify_die stuff to
get control back when the single-step trap occurs.

In other words the API for single-stepping is just the notify_die
stuff.  We don't get very formal or elaborate about APIs in the
kernel, preferring to extend a simple API when a need is shown rather
than designing an elaborate API up front that attempts to cater for
all possible needs.

And yes this is saying that itrace should use notify_die.  The
debugger_sstep hook is really only for in-kernel debuggers.  If
notify_die is not sufficient for itrace, let me know and we'll work
something else out.

Regards,
Paul.