linux-kernel - Re: [PATCH V9 3/4] riscv: ftrace: Add DYNAMIC_FTRACE_WITH_DIRECT

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Message-ID: <87ttwjjpx8.fsf@all.your.base.are.belong.to.us>
Date:   Thu, 11 May 2023 09:19:31 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     Song Shuai <suagrfillet@...il.com>, paul.walmsley@...ive.com,
        palmer@...belt.com, aou@...s.berkeley.edu, rostedt@...dmis.org,
        mhiramat@...nel.org, mark.rutland@....com, guoren@...nel.org,
        suagrfillet@...il.com, jszhang@...nel.org, e.shatokhin@...ro.com
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH V9 3/4] riscv: ftrace: Add
 DYNAMIC_FTRACE_WITH_DIRECT_CALLS support

Song Shuai <suagrfillet@...il.com> 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@...il.com>
> Tested-by: Guo Ren <guoren@...nel.org>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> ---
>  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?


Björn

[1] https://github.com/riscv-non-isa/riscv-asm-manual/blob/master/riscv-asm.md#-a-listing-of-standard-risc-v-pseudoinstructions