Messages in this thread | | | Date | Mon, 30 Mar 2020 22:29:27 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4 01/13] objtool: Remove CFI save/restore special case |
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On Mon, Mar 30, 2020 at 10:02:54PM +0200, Peter Zijlstra wrote: > On Mon, Mar 30, 2020 at 02:02:05PM -0500, Josh Poimboeuf wrote: > > However sync_core() and ftrace_regs_caller() are very different from > > each other and I find the RET_TAIL hint usage to be extremely confusing. > > I was going with the pattern: > > push target > ret > > which is an indirect tail-call that doesn't need a register. We use it > in various places. We use it here exactly because it preserves all > registers, but we use it in function-graph tracer and retprobes to > insert the return handler. But also in retpoline, because it uses the > return stack predictor, which by happy accident isn't the indirect > branch predictor. > > > For example, IRETQ isn't even a tail cail. > > It's the same indirect call, except with a bigger frame ;-) > > push # ss > push # rsp > push # flags > push # cs > push # ip > iret > > > And the need for the hint to come *before* the insn which changes the > > state is different from the other hints. > > makes sense to me... but yah.
Also, naturally, there are no instructions after RET to stick the annotation to.
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