Re: [PATCH 5/5] IA64 dynamic ftrace support

[Shaohua Li <shaohua.li@intel.com>]

On Thu, Dec 25, 2008 at 11:54:33AM +0800, Steven Rostedt wrote:
> 
> On Thu, 2008-12-25 at 09:08 +0800, Shaohua Li wrote:
> > On Thu, Dec 25, 2008 at 05:50:50AM +0800, Keith Owens wrote:
> > > On Wed, 24 Dec 2008 08:29:05 -0500, 
> > > Steven Rostedt <srostedt@redhat.com> wrote:
> > > >Yes I understand that the module and kernel code is set up differently,
> > > >PPC is pretty much the same in this aspect. I'm asking if it is easy to
> > > >change a call from the module to kernel core to another function in
> > > >kernel core?
> > > >
> > > >Question: if I have a call from the module to _mcount, how much has to
> > > >change in the set up of the registers to make it call ftrace_call
> > > >instead?  Perhaps we could link in a call to ftrace_call via the tricks
> > > >in recordmcount.pl to get the info needed to make that change?
> > > 
> > > The IA64 kernel uses the same gp register throughout, it is compiled
> > > with -mconstant-gp.  So changing the target address from one kernel
> > > function to another only requires changing the destination address in
> > > the PLT stub, no other registers are affected.
> > yes, for kernel, this is simple. Just changing the target address is ok,
> > and the change is atomic, as it's a 64-bit write. For module, it's not
> > simple. Module has different gp register against kernel. In a module,
> > _mcount must save its gp first and then jump to kernel. That's why we
> > can't directly use a jump.
> > 
> > I'm considering link some code to ftrace_call in recordmcount.pl, but
> > recordmocunt.pl is called for each file. If a module has multiple files,
> > there will be some duplicate code. Another issue how can we find the
> > code's address when ftrace to convert code to nop.
> 
> Since this still sounds like PPC actions, I'll try to show a pseudo code
> style example.
> 
> I'm assuming that a call to mcount from a module looks something like
> this:
> 
> 	save module gp
> 	load kernel gp
> 	jump to mcount (or to a mcount trampoline)
> 
> Since mcount and ftrace_caller share the same gp, could we not just
> change that jmp to ftrace_caller instead? (or to a trampoline to
> ftrace_caller as we do in PPC).
As the 25bit limit, we must use a mcount trampoline. In IA64, PLT stub
will do:
load kernel gp
jump to mcount
the PLT stub doesn't save gp, so it's not ok for the trampoline. This is
what I said We need add another trampoline code to module. Loading
module in IA64 only can add PLT stub, we need other approach for the
trampoline code.

Thanks,
Shaohua