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Message-Id: <20200724011143.a2dd2e231411e46edde2e8c4@kernel.org>
Date: Fri, 24 Jul 2020 01:11:43 +0900
From: Masami Hiramatsu <mhiramat@...nel.org>
To: Guo Ren <guoren@...nel.org>
Cc: Palmer Dabbelt <palmerdabbelt@...gle.com>,
Paul Walmsley <paul.walmsley@...ive.com>,
Oleg Nesterov <oleg@...hat.com>,
linux-riscv <linux-riscv@...ts.infradead.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Anup Patel <anup@...infault.org>, linux-csky@...r.kernel.org,
Greentime Hu <greentime.hu@...ive.com>,
Zong Li <zong.li@...ive.com>,
Patrick Stählin <me@...ki.ch>,
Björn Töpel <bjorn.topel@...il.com>,
Guo Ren <guoren@...ux.alibaba.com>,
Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH v3 6/7] riscv: Add KPROBES_ON_FTRACE supported
On Wed, 22 Jul 2020 21:31:20 +0800
Guo Ren <guoren@...nel.org> wrote:
> Hi Masami,
>
> Current riscv ftrace_caller utilize fp(s0) - 8 in stack to get ra of
> function, eg:
> foo:
> 2bb0: 7119 addi sp,sp,-128
> 2bb2: f8a2 sd s0,112(sp)
> 2bb4: fc86 sd ra,120(sp)
> ...
> 2bc4: 0100 addi s0,sp,128
> ...
> 0000000000002bca <.LVL828>:
> 2bca: 00000097 auipc ra,0x0
> 2bce: 000080e7 jalr ra # 2bca <.LVL828> //_mcount
>
> So just put two nops before prologue of function isn't enough, because
> riscv don't like arm64 which could use x9-x18 reserved regs to pass
> ra(x30).
> | mov x9, x30
> | bl <ftrace-entry>
> If the benefit is just making a kprobe on function symbol address to
> prevent disassembling, I'll delay this feature.
I recommend that. This feature has to involve ftrace and gcc, so
it is better to split it from this series.
>
>
> I also have a look at HAVE_FENTRY & HAVE_NOP_MCOUNT. Seems it just
> avoid using scripts/recordmcount.pl script and directly generate nops
> for _mcount.
Right.
> It's different from -fpatchable-function-entry=2 which generating nops
> before function prologue in arm64, isn't it?
Yes, fentry is for x86, but -fpatchable-function-entry=2 is making a
placeholder with nop at the entry of the functions for direct patching.
Thank you,
>
> On Tue, Jul 21, 2020 at 9:27 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > On Wed, 15 Jul 2020 00:24:54 +0800
> > Guo Ren <guoren@...nel.org> wrote:
> >
> > > Thx Masami,
> > >
> > > On Tue, Jul 14, 2020 at 7:38 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > >
> > > > On Mon, 13 Jul 2020 23:39:21 +0000
> > > > guoren@...nel.org wrote:
> > > >
> > > > > From: Guo Ren <guoren@...ux.alibaba.com>
> > > > >
> > > > > This patch adds support for kprobes on ftrace call sites to avoids
> > > > > much of the overhead with regular kprobes. Try it with simple
> > > > > steps:
> > > > >
> > > > > 1. Get _do_fork ftrace call site.
> > > > > Dump of assembler code for function _do_fork:
> > > > > 0xffffffe00020af64 <+0>: addi sp,sp,-128
> > > > > 0xffffffe00020af66 <+2>: sd s0,112(sp)
> > > > > 0xffffffe00020af68 <+4>: sd ra,120(sp)
> > > > > 0xffffffe00020af6a <+6>: addi s0,sp,128
> > > > > 0xffffffe00020af6c <+8>: sd s1,104(sp)
> > > > > 0xffffffe00020af6e <+10>: sd s2,96(sp)
> > > > > 0xffffffe00020af70 <+12>: sd s3,88(sp)
> > > > > 0xffffffe00020af72 <+14>: sd s4,80(sp)
> > > > > 0xffffffe00020af74 <+16>: sd s5,72(sp)
> > > > > 0xffffffe00020af76 <+18>: sd s6,64(sp)
> > > > > 0xffffffe00020af78 <+20>: sd s7,56(sp)
> > > > > 0xffffffe00020af7a <+22>: mv s4,a0
> > > > > 0xffffffe00020af7c <+24>: mv a0,ra
> > > > > 0xffffffe00020af7e <+26>: nop <<<<<<<< here!
> > > > > 0xffffffe00020af82 <+30>: nop
> > > > > 0xffffffe00020af86 <+34>: ld s3,0(s4)
> > > > >
> > > > > 2. Set _do_fork+26 as the kprobe.
> > > > > echo 'p:myprobe _do_fork+26 dfd=%a0 filename=%a1 flags=%a2 mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
> > > > > echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
> > > > > cat /sys/kernel/debug/tracing/trace
> > > > > tracer: nop
> > > > >
> > > > > entries-in-buffer/entries-written: 3/3 #P:1
> > > > >
> > > > > _-----=> irqs-off
> > > > > / _----=> need-resched
> > > > > | / _---=> hardirq/softirq
> > > > > || / _--=> preempt-depth
> > > > > ||| / delay
> > > > > TASK-PID CPU# |||| TIMESTAMP FUNCTION
> > > > > | | | |||| | |
> > > > > sh-87 [000] .... 551.557031: myprobe: (_do_fork+0x1a/0x2e6) dfd=0xffffffe00020af7e filename=0xffffffe00020b34e flags=0xffffffe00101e7c0 mode=0x20af86ffffffe0
> > > > >
> > > > > cat /sys/kernel/debug/kprobes/list
> > > > > ffffffe00020af7e k _do_fork+0x1a [FTRACE]
> > > > > ^^^^^^
> > > >
> > > > Hmm, this seems fentry is not supported on RISC-V yet. But anyway,
> > > > it will be useful for users (if they can find the offset).
> > >
> > > Seems only x86 & ⬆️90 use fentry，can you elaborate more about fentry's
> > > benefit and how the user could set kprobe on ftrace call site without
> > > disassemble?
> >
> > On x86, the fentry replaces the mcount with just one call instruction, without
> > saving any arguments. This means all probes which are puts on the address of
> > target symbol, are automatically using ftrace. IOW, all probes on _do_fork+0
> > will use ftrace. We don't need any disassembling.
> >
> > I think if RISC-V already support "-fpatchable-function-entry=2" option on
> > GCC, you can easily enable it as same as arm64. See https://lkml.org/lkml/2019/6/18/648
> >
> > Thank you,
> >
> > --
> > Masami Hiramatsu <mhiramat@...nel.org>
>
>
>
> --
> Best Regards
> Guo Ren
>
> ML: https://lore.kernel.org/linux-csky/
--
Masami Hiramatsu <mhiramat@...nel.org>
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