[PATCH V11 0/5] riscv: Optimize function trace

[Björn Töpel]

Palmer Dabbelt <palmer at rivosinc.com> writes:

> On Wed, 12 Jul 2023 11:11:08 PDT (-0700), bjorn at kernel.org wrote:
>> Song Shuai <suagrfillet at gmail.com> writes:
>>
>> [...]
>>
>>> Add WITH_DIRECT_CALLS support [3] (patch 3, 4)
>>> ==============================================
>>
>> We've had some offlist discussions, so here's some input for a wider
>> audience! Most importantly, this is for Palmer, so that this series is
>> not merged until a proper BPF trampoline fix is in place.
>>
>> Note that what's currently usable from BPF trampoline *works*. It's
>> when this series is added that it breaks.
>>
>> TL;DR This series adds DYNAMIC_FTRACE_WITH_DIRECT_CALLS, which enables
>> fentry/fexit BPF trampoline support. Unfortunately the
>> fexit/BPF_TRAMP_F_SKIP_FRAME parts of the RV BPF trampoline breaks
>> with this addition, and need to be addressed *prior* merging this
>> series. An easy way to reproduce, is just calling any of the kselftest
>> tests that uses fexit patching.
>>
>> The issue is around the nop seld, and how a call is done; The nop sled
>> (patchable-function-entry) size changed from 16B to 8B in commit
>> 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to half"), but
>> BPF code still uses the old 16B. So it'll work for BPF programs, but not
>> for regular kernel functions.
>>
>> An example:
>>
>>   | ffffffff80fa4150 <bpf_fentry_test1>:
>>   | ffffffff80fa4150:       0001                    nop
>>   | ffffffff80fa4152:       0001                    nop
>>   | ffffffff80fa4154:       0001                    nop
>>   | ffffffff80fa4156:       0001                    nop
>>   | ffffffff80fa4158:       1141                    add     sp,sp,-16
>>   | ffffffff80fa415a:       e422                    sd      s0,8(sp)
>>   | ffffffff80fa415c:       0800                    add     s0,sp,16
>>   | ffffffff80fa415e:       6422                    ld      s0,8(sp)
>>   | ffffffff80fa4160:       2505                    addw    a0,a0,1
>>   | ffffffff80fa4162:       0141                    add     sp,sp,16
>>   | ffffffff80fa4164:       8082                    ret
>>
>> is patched to:
>>
>>   | ffffffff80fa4150:  f70c0297                     auipc   t0,-150208512
>>   | ffffffff80fa4154:  eb0282e7                     jalr    t0,t0,-336
>>
>> The return address to bpf_fentry_test1 is stored in t0 at BPF
>> trampoline entry. Return to the *parent* is in ra. The trampline has
>> to deal with this.
>>
>> For BPF_TRAMP_F_SKIP_FRAME/CALL_ORIG, the BPF trampoline will skip too
>> many bytes, and not correctly handle parent calls.
>>
>> Further; The BPF trampoline currently has a different way of patching
>> the nops for BPF programs, than what ftrace does. That should be changed
>> to match what ftrace does (auipc/jalr t0).
>>
>> To summarize:
>>  * Align BPF nop sled with patchable-function-entry: 8B.
>>  * Adapt BPF trampoline for 8B nop sleds.
>>  * Adapt BPF trampoline t0 return, ra parent scheme.
>
> Thanks for digging into this one, I agree we need to sort out the BPF 
> breakages before we merge this.  Sounds like there's a rabbit hole here, 
> but hopefully we can get it sorted out.
>
> I've dropped this from patchwork and such, as we'll need at least 
> another spin.

Palmer,

The needed BPF patch is upstream in the bpf-next tree, and has been for
a couple of weeks.

I think this series is a candidate for RISC-V -next! It would help
RISC-V BPF a lot in terms of completeness.


Björn