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Message-ID: <20200330190205.k5ssixd5hpshpjjq@treble>
Date: Mon, 30 Mar 2020 14:02:05 -0500
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
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 07:02:00PM +0200, Peter Zijlstra wrote:
> Subject: objtool: Implement RET_TAIL hint
>
> This replaces the SAVE/RESTORE hints with a RET_TAIL hint that applies to:
>
> - regular RETURN and sibling calls (which are also function exists)
> it allows the stack-frame to be off by one word, ie. it allows a
> return-tail-call.
>
> - EXCEPTION_RETURN (a new INSN_type that splits IRET out of
> CONTEXT_SWITCH) and here it denotes a return to self by having it
> consume arch_exception_frame_size bytes off the stack and continuing.
>
> Apply this hint to ftrace_64.S and sync_core(), the two existing users
> of the SAVE/RESTORE hints.
>
> For ftrace_64.S we split the return path and make sure the
> ftrace_epilogue call is seen as a sibling/tail-call turning it into it's
> own function.
>
> By splitting the return path every instruction has a unique stack setup
> and ORC can generate correct unwinds (XXX check if/how the ftrace
> trampolines map into the ORC). Then employ the RET_TAIL hint to the
> tail-call exit that has the direct-call (orig_eax) return-tail-call on.
>
> For sync_core() annotate the IRET with RET_TAIL to mark it as a
> control-flow NOP that consumes the exception frame.
I do like the idea to get rid of SAVE/RESTORE altogether. And it's nice
to make that ftrace code unwinder-deterministic.
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.
For example, IRETQ isn't even a tail cail.
And the need for the hint to come *before* the insn which changes the
state is different from the other hints.
And now objtool has to know the arch exception stack size because of a
single code site.
And for a proper tail call, the stack should be empty. I don't
understand the +8 thing in has_modified_stack_frame(). It seems
hard-coded for the weird ftrace case, rather than for tail calls in
general (which should already work as designed).
How about a more general hint like UNWIND_HINT_ADJUST?
For sync_core(), after the IRETQ:
UNWIND_HINT_ADJUST sp_add=40
And ftrace_regs_caller_ret could have:
UNWIND_HINT_ADJUST sp_add=8
--
Josh
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