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 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.
It's different from -fpatchable-function-entry=2 which generating nops
before function prologue in arm64, isn't it?
On Tue, Jul 21, 2020 at 9:27 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Wed, 15 Jul 2020 00:24:54 +0800
> Guo Ren <guoren@xxxxxxxxxx> wrote:
>
> > Thx Masami,
> >
> > On Tue, Jul 14, 2020 at 7:38 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> > >
> > > On Mon, 13 Jul 2020 23:39:21 +0000
> > > guoren@xxxxxxxxxx wrote:
> > >
> > > > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxx>
--
Best Regards
Guo Ren
ML: https://lore.kernel.org/linux-csky/
