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@xxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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