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    SubjectRe: [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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