Re: [PATCH V9 3/4] riscv: ftrace: Add DYNAMIC_FTRACE_WITH_DIRECT_CALLS support

[Guo Ren]

On Thu, May 11, 2023 at 9:30 PM Evgenii Shatokhin <e.shatokhin@xxxxxxxxx> wrote:
>
> Hi,
>
> On 11.05.2023 10:19, Björn Töpel wrote:
> > Song Shuai <suagrfillet@xxxxxxxxx> writes:
> >
> >> This patch adds DYNAMIC_FTRACE_WITH_DIRECT_CALLS support for RISC-V.
> >>
> >> select the DYNAMIC_FTRACE_WITH_DIRECT_CALLS to provide the
> >> register_ftrace_direct[_multi] interfaces allowing users to register
> >> the customed trampoline (direct_caller) as the mcount for one or
> >> more target functions. And modify_ftrace_direct[_multi] are also
> >> provided for modifying direct_caller.
> >>
> >> To make the direct_caller and the other ftrace hooks (eg. function/fgraph
> >> tracer, k[ret]probes) co-exist, a temporary register is nominated to
> >> store the address of direct_caller in ftrace_regs_caller. After the
> >> setting of the address direct_caller by direct_ops->func and the
> >> RESTORE_REGS in ftrace_regs_caller, direct_caller will be jumped to
> >> by the `jr` inst.
> >>
> >> Signed-off-by: Song Shuai <suagrfillet@xxxxxxxxx>
> >> Tested-by: Guo Ren <guoren@xxxxxxxxxx>
> >> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> >> ---
> >>   arch/riscv/Kconfig              |  1 +
> >>   arch/riscv/include/asm/ftrace.h |  8 ++++++++
> >>   arch/riscv/kernel/mcount-dyn.S  | 10 ++++++++++
> >>   3 files changed, 19 insertions(+)
> >>
> >> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> >> index e0632493482f..fdf0b219a02c 100644
> >> --- a/arch/riscv/Kconfig
> >> +++ b/arch/riscv/Kconfig
> >> @@ -144,6 +144,7 @@ config RISCV
> >>        select UACCESS_MEMCPY if !MMU
> >>        select ZONE_DMA32 if 64BIT
> >>        select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && MMU && (CLANG_SUPPORTS_DYNAMIC_FTRACE || GCC_SUPPORTS_DYNAMIC_FTRACE)
> >> +     select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> >>        select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
> >>        select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
> >>        select HAVE_FUNCTION_GRAPH_TRACER
> >> diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> >> index 84f856a3286e..84904c1e4369 100644
> >> --- a/arch/riscv/include/asm/ftrace.h
> >> +++ b/arch/riscv/include/asm/ftrace.h
> >> @@ -114,6 +114,14 @@ struct ftrace_regs;
> >>   void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
> >>                       struct ftrace_ops *op, struct ftrace_regs *fregs);
> >>   #define ftrace_graph_func ftrace_graph_func
> >> +
> >> +static inline void
> >> +__arch_ftrace_set_direct_caller(struct pt_regs *regs, unsigned long addr)
> >> +{
> >> +             regs->t1 = addr;
> >> +}
> >> +#define arch_ftrace_set_direct_caller(fregs, addr) \
> >> +     __arch_ftrace_set_direct_caller(&(fregs)->regs, addr)
> >>   #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> >>
> >>   #endif /* __ASSEMBLY__ */
> >> diff --git a/arch/riscv/kernel/mcount-dyn.S b/arch/riscv/kernel/mcount-dyn.S
> >> index f26e9f6e2fed..9d405baedb52 100644
> >> --- a/arch/riscv/kernel/mcount-dyn.S
> >> +++ b/arch/riscv/kernel/mcount-dyn.S
> >> @@ -231,6 +231,7 @@ ENDPROC(ftrace_caller)
> >>
> >>   #else /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> >>   ENTRY(ftrace_regs_caller)
> >> +     move    t1, zero
> >
> > Please use "mv", and not "move" [1].
> >
> >>        SAVE_ABI_REGS 1
> >>        PREPARE_ARGS
> >>
> >> @@ -239,7 +240,10 @@ ftrace_regs_call:
> >>        call    ftrace_stub
> >>
> >>        RESTORE_ABI_REGS 1
> >> +     bnez    t1,.Ldirect
> >>        jr t0
> >> +.Ldirect:
> >> +     jr t1
> >
> > Again, while you're doing changes here, please try to align op/operands.
> >
> > Wearing my BPF hat, I'm happy to finally get DIRECT_CALLS support!
> >
> > This does not take the WITH_CALL_OPS approach Mark suggested in the v7
> > threads, but given that text patching story on RISC-V is still a bit sad
> > (inconsistency in the RV tree, no specification, cannot work with
> > preempt, ...) I'd say this approach is OK for now, and we can change to
> > WITH_CALL_OPS later in a wider "let's improve RISC-V textpatching" work.
> >
> > Thoughts?
>
> The WITH_CALL_OPS approach seems to need much more time and effort, so,
> yes, I'd also use this implementation of DIRECT_CALLS for now. Other
> improvements could wait for the future "big patching rework".
I agree to make DIRECT_CALLS merged first, WITH_CALL_OPS is another
"big patching rework".

>
> FWIW, the implementation of kprobes for RISC-V has been improving
> gradually too, not everything was done in the first very patchset, but
> it was usable nonetheless.
>
> I have not tested this particular version of this
> DYNAMIC_FTRACE_WITH_DIRECT_CALLS series, only some previous ones - there
> were no outstanding problems there. The code looks sane to me.
>
> >
> >
> > Björn
> >
> > [1] https://github.com/riscv-non-isa/riscv-asm-manual/blob/master/riscv-asm.md#-a-listing-of-standard-risc-v-pseudoinstructions
> >
>
> Regards,
> Evgenii
>
>


-- 
Best Regards
 Guo Ren