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Message-ID: <mhng-a2c88f43-3cf7-4caa-8e4a-b0fc9d7e4628@palmer-ri-x1c9a>
Date: Wed, 12 Jul 2023 11:26:09 -0700 (PDT)
From: Palmer Dabbelt <palmer@...osinc.com>
To: bjorn@...nel.org
CC: suagrfillet@...il.com, Paul Walmsley <paul.walmsley@...ive.com>,
aou@...s.berkeley.edu, rostedt@...dmis.org, mhiramat@...nel.org,
Mark Rutland <mark.rutland@....com>, guoren@...nel.org,
suagrfillet@...il.com, Bjorn Topel <bjorn@...osinc.com>,
jszhang@...nel.org, Conor Dooley <conor.dooley@...rochip.com>,
pulehui@...wei.com, linux-riscv@...ts.infradead.org,
linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
songshuaishuai@...ylab.org, bpf@...r.kernel.org
Subject: Re: [PATCH V11 0/5] riscv: Optimize function trace
On Wed, 12 Jul 2023 11:11:08 PDT (-0700), bjorn@...nel.org wrote:
> Song Shuai <suagrfillet@...il.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.
> Cheers,
> Björn
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