linux-kernel - Re: [PATCH v4 01/13] objtool: Remove CFI save/restore special case

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Date:   Mon, 30 Mar 2020 22:29:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        mhiramat@...nel.org, mbenes@...e.cz,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v4 01/13] objtool: Remove CFI save/restore special case

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.