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

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Message-ID: <50d56c9c-49c7-8d93-6698-3d0e90d89c54@yadro.com>
Date:   Thu, 11 May 2023 16:30:44 +0300
From:   Evgenii Shatokhin <e.shatokhin@...ro.com>
To:     Björn Töpel <bjorn@...nel.org>,
        Song Shuai <suagrfillet@...il.com>
CC:     <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <linux-trace-kernel@...r.kernel.org>, <paul.walmsley@...ive.com>,
        <rostedt@...dmis.org>, <palmer@...belt.com>,
        <aou@...s.berkeley.edu>, <mhiramat@...nel.org>,
        <mark.rutland@....com>, <guoren@...nel.org>, <jszhang@...nel.org>
Subject: Re: [PATCH V9 3/4] riscv: ftrace: Add
 DYNAMIC_FTRACE_WITH_DIRECT_CALLS support

Hi,

On 11.05.2023 10:19, Björn Töpel wrote:
> 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?

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".

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