Messages in this thread | | | Date | Tue, 24 Mar 2009 23:48:58 +0100 | From | Frederic Weisbecker <> | Subject | Re: Anyone working on ftrace function graph support on ARM? |
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On Tue, Mar 24, 2009 at 06:29:03PM -0400, Abhishek Sagar wrote: > Frederic Weisbecker wrote: > > Yes, ie: > > > > _Before jumping to the function entry hook, you must save > > the arguments for the traced function on the stack. > > On x86, its eax, edx and ecx. > > On arm, it will be r0-r3. > > Then you have to transmit the address of the traced function > > (it's on r14) and it's parent (must rely on fp for that). > > Then you call the entry hook and restore the old scratch/arg > > registers. > > > Instead of just restoring the old backed-up args, lr can be fixed up > inside the entry hook to point to the return hook. So when the traced > function returns, it actually returns to the return hook (where we can > restore the original return address). This means that > -finstrument-functions is not required at all. This is analogous to how > kretprobes work. The only difference here is that instead of planting a > kprobe at the function entry and redirecting the function return to the > profiling exit routine, we can use mcount. This is slightly more > complicated to implement but can be a very efficient alternative to > kretprobes. > -- > Abhishek >
Indeed, you need to override lr, that even the only solution. I was still thinking in an x86 way with its on stack return address.
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