On Mon, Dec 11, 2017 at 10:15:58AM -0800, Palmer Dabbelt wrote: > On Wed, 06 Dec 2017 18:31:10 PST (-0800), noner...@gmail.com wrote:
Hi Palmer, I forgot to explain this section in the previous reply: > > +ENTRY(_mcount) > > + la t4, ftrace_stub > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > + la t0, ftrace_graph_return > > + ld t1, 0(t0) > > + bne t1, t4, do_ftrace_graph_caller > > + > > + la t3, ftrace_graph_entry > > + ld t2, 0(t3) > > + la t6, ftrace_graph_entry_stub > > + bne t2, t6, do_ftrace_graph_caller > > +#endif > > + la t3, ftrace_trace_function > > + ld t5, 0(t3) > > + bne t5, t4, do_trace > > + ret > > * You can save an instruction when addressing by using somethingl like "ld > t1, ftrace_graph_return" instead of "la t0, ftrace_graph_return; ld t1 > 0(t0)". > There are three "la-ld" instruction pairs for loading ftrace_graph_return, ftrace_graph_entry, and ftrace_trace_function. All of them are function pointers in C. The problem here is that, if we applied an "ld" inst. to a function pointer, we would have loaded the content, which would be the first 8 bytes in the function, rather than the address of the target function that the function pointer stored before. In brief, the logic of the "la-ld" pairs should be fine. Alan
