Messages in this thread | | | Date | Sat, 15 Jul 2023 17:10:26 +0800 | Subject | Re: [PATCH V11 0/5] riscv: Optimize function trace | From | Pu Lehui <> |
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On 2023/7/13 2:11, Björn Töpel wrote: > Song Shuai <suagrfillet@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 Björn, I make a adaptation as follows, looking forward to your review.
https://lore.kernel.org/bpf/20230715090137.2141358-1-pulehui@huaweicloud.com/
> > Cheers, > Björn > >
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