Re: [PATCH v4 0/8] Add OPTPROBES feature on RISCV

From: "liaochang (A)" <liaochang1@huawei.com>
To: Xim <chenguokai17@mails.ucas.ac.cn>, "Björn Töpel" <bjorn@kernel.org>
Cc: <paul.walmsley@sifive.com>, <palmer@dabbelt.com>,
	<aou@eecs.berkeley.edu>, <rostedt@goodmis.org>,
	<mingo@redhat.com>, <sfr@canb.auug.org.au>,
	<linux-riscv@lists.infradead.org>, <linux-kernel@vger.kernel.org>,
	Liao Chang <liaoclark@163.com>
Subject: Re: [PATCH v4 0/8] Add OPTPROBES feature on RISCV
Date: Tue, 8 Nov 2022 19:33:54 +0800	[thread overview]
Message-ID: <598bc40d-e826-f9cc-14fd-f4570051f4c6@huawei.com> (raw)
In-Reply-To: <9A705974-A007-45E2-BC5D-A7E90821A258@mails.ucas.ac.cn>

在 2022/11/8 19:04, Xim 写道:
> Hi Björn,
> 
> Thanks for your great review! Some explanations below.
> 
>> 2022年11月8日 00:54，Björn Töpel <bjorn@kernel.org> 写道：
>>
>> Have you run the series on real hardware, or just qemu?
> 
> Currently only qemu tests are made, I will try to test it on a FPGA real hardware soon.
> 
>> AFAIU, the algorithm only tracks registers that are *in use*. You are
>> already scanning the whole function (next patch). What about the caller
>> saved registers that are *not* used by the function in the probe range?
>> Can those, potentially unused, regs be used?
> 
> Great missing part! I have made a static analyzation right upon receiving this mail.
> The result shows that this newly purposed idea reaches about the same
> success rate on my test set (rv64 defconf with RVI only) while when combined,
> they can reach a higher success rate, 1/3 above their baseline. A patch that
> includes this strategy will be sent soon.
>>
>>> +static void arch_find_register(unsigned long start, unsigned long end,
>>
>> Nit; When I see "arch_" I think it's functionality that can be
>> overridden per-arch. This is not the case, but just a helper for RV.
> 
> It can be explained from two aspects. First, it can be extended to most RISC
> archs, which can be extracted into the common flow of Kprobe. Second, it is indeed
> a internal helper for now, so I will correct the name in the next version.
> 
>>> static void find_free_registers(struct kprobe *kp, struct optimized_kprobe *op,
>>> -				int *rd1, int *rd2)
>>> +				int *rd, int *ra)
>>
>> Nit; Please get rid of this code churn, just name the parameters
>> correctly on introduction in the previous patch.
> 
> Will be fixed.
> 
>>> +	*rd = ((kw | ow) == 1UL) ? 0 : __builtin_ctzl((kw | ow) & ~1UL);
>>> +	*ra = (kw == 1UL) ? 0 : __builtin_ctzl(kw & ~1UL);
>>
>> Hmm, __builtin_ctzl is undefined for 0, right? Can that be triggered
>> here?

This corner case has been taken into account, look these condition parts,
if kw == 1UL this expression will return 0 directly, no chance to invoke __builtin_ctzl.

Thanks.

> 
> Will be fixed.
> 
> Regards,
> Guokai Chen
> 

-- 
BR,
Liao, Chang

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