On Thu, May 09, 2019 at 10:14:31AM +0200, Peter Zijlstra wrote:
>
> Right; I already fixed that in my patch changing i386's pt_regs.
>
> But what I'd love to do is something like the belwo patch, and make all
> the trampolines (very much including ftrace) use that. Such that we then
> only have 1 copy of this magic (well, 2 because x86_64 also needs an
> implementation of this of course).
>
> Changing ftrace over to this would be a little more work but it can
> easily chain things a little to get its original context back:
>
> ENTRY(ftrace_regs_caller)
> GLOBAL(ftrace_regs_func)
> push ftrace_stub
> push ftrace_regs_handler
Note, ftrace_stub is dynamically modified to remove any indirect calls.
> jmp call_to_exception_trampoline
> END(ftrace_regs_caller)
>
> typedef void (*ftrace_func_t)(unsigned long, unsigned long, struct ftrace_op *, struct pt_regs *);
>
> struct ftrace_regs_stack {
> ftrace_func_t func;
> unsigned long parent_ip;
> };
>
> void ftrace_regs_handler(struct pr_regs *regs)
> {
> struct ftrace_regs_stack *st = (void *)regs->sp;
> ftrace_func_t func = st->func;
>
> regs->sp += sizeof(long); /* pop func */
>
> func(regs->ip, st->parent_ip, function_trace_op, regs);
I try very hard to limit all indirect function calls from the function tracing
path, as they do add noticeable overhead.
-- Steve
> }
>
> Hmm? I didn't look into the function_graph thing, but I imagine it can
> be added without too much pain.
>
