Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

["Björn Töpel" <bjorn@kernel.org>]

Mark,

Mark Rutland <mark.rutland@arm.com> writes:

>> A) Use auipc/jalr, only patch jalr to take us to a common
>>    dispatcher/trampoline
>>   
>>  | <func_trace_target_data_8B> # probably on a data cache-line != func .text to avoid ping-pong
>>  | ...
>>  | func:
>>  |   ...make sure ra isn't messed up...
>>  |   aupic
>>  |   nop <=> jalr # Text patch point -> common_dispatch
>>  |   ACTUAL_FUNC
>>  | 
>>  | common_dispatch:
>>  |   load <func_trace_target_data_8B> based on ra
>>  |   jalr
>>  |   ...
>> 
>> The auipc is never touched, and will be overhead. Also, we need a mv to
>> store ra in a scratch register as well -- like Arm. We'll have two insn
>> per-caller overhead for a disabled caller.
>
> Is the AUIPC a significant overhead? IIUC that's similar to Arm's ADRP, and I'd
> have expected that to be pretty cheap.

No, reg-to-reg moves are dirt cheap in my book.

> IIUC your JALR can choose which destination register to store the return
> address in, and if so, you could leave the original ra untouched (and recover
> that in the common trampoline). Have I misunderstood that?
>
> Maybe that doesn't play nicely with something else?

No, you're right, we can link to another register, and shave off an
instruction. I can imagine that some implementation prefer x1/x5 for
branch prediction reasons, but that's something that we can measure on. 

So, 1-2 movs + nop are unconditionally executed on the disabled case.
(1-2 depending on the ra save/jalr reg strategy).

>> B) Use jal, which can only take us +/-1M, and requires multiple
>>    dispatchers (and tracking which one to use, and properly distribute
>>    them. Ick.)
>> 
>>  | <func_trace_target_data_8B> # probably on a data cache-line != func .text to avoid ping-pong
>>  | ...
>>  | func:
>>  |   ...make sure ra isn't messed up...
>>  |   nop <=> jal # Text patch point -> within_1M_to_func_dispatch
>>  |   ACTUAL_FUNC
>>  | 
>>  | within_1M_to_func_dispatch:
>>  |   load <func_trace_target_data_8B> based on ra
>>  |   jalr
>> 
>> C) Use jal, which can only take us +/-1M, and use a per-function
>>    trampoline requires multiple dispatchers (and tracking which one to
>>    use). Blows up text size A LOT.
>> 
>>  | <func_trace_target_data_8B> # somewhere, but probably on a different cacheline than the .text to avoid ping-ongs
>>  | ...
>>  | per_func_dispatch
>>  |   load <func_trace_target_data_8B> based on ra
>>  |   jalr
>>  | func:
>>  |   ...make sure ra isn't messed up...
>>  |   nop <=> jal # Text patch point -> per_func_dispatch
>>  |   ACTUAL_FUNC
>
> Beware that with option (C) you'll need to handle that in your unwinder for
> RELIABLE_STACKTRACE. If you don't have a symbol for per_func_dispatch (or
> func_trace_target_data_8B), PC values within per_func_dispatch would be
> symbolized as the prior function/data.

Good point (but I don't like C much...)!

>> It's a bit sad that we'll always have to have a dispatcher/trampoline,
>> but it's still better than stop_machine(). (And we'll need a fencei IPI
>> as well, but only one. ;-))
>> 
>> Today, I'm leaning towards A (which is what Mark suggested, and also
>> Robbin).. Any other options?
>
> Assuming my understanding of JALR above is correct, I reckon A is the nicest
> option out of A/B/C.

Yes! +1!


Björn

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