Björn Töpel <bj...@kernel.org> writes:
> Puranjay Mohan <puranja...@gmail.com> writes:
>
>> Björn Töpel <bj...@kernel.org> writes:
>>
>>>
>>> Hmm, depending on RISC-V's CMODX path, the pro/cons CALL_OPS vs dynamic
>>> trampolines changes quite a bit.
>>>
>>> The more I look at the pains of patching two instruction ("split
>>> immediates"), the better "patch data" + one insn patching look.
>>
>> I was looking at how dynamic trampolines would be implemented for RISC-V.
>>
>> With CALL-OPS we need to patch the auipc+jalr at function entry only, the
>> ops pointer above the function can be patched atomically.
>>
>> With a dynamic trampoline we need a auipc+jalr pair at function entry to jump
>> to the trampoline and then another auipc+jalr pair to jump from trampoline to
>> ops->func. When the ops->func is modified, we would need to update the
>> auipc+jalr at in the trampoline.
>>
>> So, I am not sure how to move forward here, CALL-OPS or Dynamic trampolines?
>
> Yeah. Honestly, we need to figure out the patching story prior
> choosing the path, so let's start there.
>
> After reading Mark's reply, and discussing with OpenJDK folks (who does
> the most crazy text patching on all platforms), having to patch multiple
> instructions (where the address materialization is split over multiple
> instructions) is a no-go. It's just a too big can of worms. So, if we
> can only patch one insn, it's CALL_OPS.
>
> A couple of options (in addition to Andy's), and all require a
> per-function landing address ala CALL_OPS) tweaking what Mark is doing
> on Arm (given the poor branch range).
>
> ...and maybe we'll get RISC-V rainbows/unicorns in the future getting
> better reach (full 64b! ;-)).
>
> 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.
>
> 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
Brendan proposed yet another option, "in-function dispatch":
D)
| <func_trace_target_data_8B_per_function> # idk somewhere
| ...
| func:
| mv tmp1, ra
| aupic tmp2, <func_trace_target_data_8B_per_function:upper>
| mv tmp3, zero <=> ld tmp3,
tmp2<func_trace_target_data_8B_per_function:lower>
| nop <=> jalr ra, tmp3
| ACTUAL_FUNC
There are 4 CMODX possiblities:
mv, nop: fully disabled, no problems
mv, jalr: We will jump to zero. We would need to have the inst
page/access fault handler take care of this case. Especially
if we align the instructions so that they can be patched
together, being interrupted in the middle and taking this
path will be rare.
ld, nop: no problems
ld, jalr: fully enabled, no problems
Patching is a 64b store/sd, and we only need a fence.i at the end, since
we can handle all 4 possibilities.
For the disabled case we'll have:
A) mv, aupic, nop
D) mv, aupic, mv, nop.
Puranjay, I've flipped. Let's go Mark's CALL_OPS together with a new
text patch mechanism w/o stop_machine().
Björn
