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Subject: Re: [PATCH v2] arm64: kprobe: Enable OPTPROBE for arm64
To:     Masami Hiramatsu <mhiramat@kernel.org>,
        Linuxarm <linuxarm@huawei.com>
CC:     <catalin.marinas@arm.com>, <will@kernel.org>,
        <naveen.n.rao@linux.ibm.com>, <anil.s.keshavamurthy@intel.com>,
        <davem@davemloft.net>, <linux-arm-kernel@lists.infradead.org>,
        <song.bao.hua@hisilicon.com>, <prime.zeng@hisilicon.com>,
        <robin.murphy@arm.com>, <f.fangjian@huawei.com>,
        <linux-kernel@vger.kernel.org>
References: <20210804060209.95817-1-liuqi115@huawei.com>
 <20210805105401.4acd3217c566b4e3933f355c@kernel.org>
From:   "liuqi (BA)" <liuqi115@huawei.com>
Message-ID: <d3e0c9ee-19b5-8042-d251-05348e8ac49e@huawei.com>
Date:   Thu, 5 Aug 2021 17:25:17 +0800
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Hi Masami,

On 2021/8/5 9:54, Masami Hiramatsu wrote:
> On Wed, 4 Aug 2021 14:02:09 +0800
> Qi Liu <liuqi115@huawei.com> wrote:
> 
>> This patch introduce optprobe for ARM64. In optprobe, probed
>> instruction is replaced by a branch instruction to detour
>> buffer. Detour buffer contains trampoline code and a call to
>> optimized_callback(). optimized_callback() calls opt_pre_handler()
>> to execute kprobe handler.
>>
>> Limitations:
>> - We only support !CONFIG_RANDOMIZE_MODULE_REGION_FULL case to
>> guarantee the offset between probe point and kprobe pre_handler
>> is not larger than 128MiB.
>>
>> Performance of optprobe on Hip08 platform is test using kprobe
>> example module[1] to analyze the latency of a kernel function,
>> and here is the result:
>>
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/kprobes/kretprobe_example.c
>>
>> kprobe before optimized:
>> [280709.846380] do_empty returned 0 and took 1530 ns to execute
>> [280709.852057] do_empty returned 0 and took 550 ns to execute
>> [280709.857631] do_empty returned 0 and took 440 ns to execute
>> [280709.863215] do_empty returned 0 and took 380 ns to execute
>> [280709.868787] do_empty returned 0 and took 360 ns to execute
>> [280709.874362] do_empty returned 0 and took 340 ns to execute
>> [280709.879936] do_empty returned 0 and took 320 ns to execute
>> [280709.885505] do_empty returned 0 and took 300 ns to execute
>> [280709.891075] do_empty returned 0 and took 280 ns to execute
>> [280709.896646] do_empty returned 0 and took 290 ns to execute
>> [280709.902220] do_empty returned 0 and took 290 ns to execute
>> [280709.907807] do_empty returned 0 and took 290 ns to execute
>>
>> optprobe:
>> [ 2965.964572] do_empty returned 0 and took 90 ns to execute
>> [ 2965.969952] do_empty returned 0 and took 80 ns to execute
>> [ 2965.975332] do_empty returned 0 and took 70 ns to execute
>> [ 2965.980714] do_empty returned 0 and took 60 ns to execute
>> [ 2965.986128] do_empty returned 0 and took 80 ns to execute
>> [ 2965.991507] do_empty returned 0 and took 70 ns to execute
>> [ 2965.996884] do_empty returned 0 and took 70 ns to execute
>> [ 2966.002262] do_empty returned 0 and took 80 ns to execute
>> [ 2966.007642] do_empty returned 0 and took 70 ns to execute
>> [ 2966.013020] do_empty returned 0 and took 70 ns to execute
>> [ 2966.018400] do_empty returned 0 and took 70 ns to execute
>> [ 2966.023779] do_empty returned 0 and took 70 ns to execute
>> [ 2966.029158] do_empty returned 0 and took 70 ns to execute
>>
>> Signed-off-by: Qi Liu <liuqi115@huawei.com>
>>
>> ---
>>
>> Changes since V1:
>> - Address the comments from Masami, checks for all branch instructions, and
>> use aarch64_insn_patch_text_nosync() instead of aarch64_insn_patch_text()
>> in each probe.
> 
> Is it safe for the multicore system? If it is safe because it modifies
> just one instruction (modifying 32bit in atomic), I understand it.

Seems raw_spin_lock_irqsave is used in aarch64_insn_patch_text_nosync() 
and spinlock could support a protection in multicore system.
> BTW, anyway, you should use _nosync() variant in arch_prepare_optimized_kprobe()
> too, beacause the optprobe insn buffer is not touched until the probed instruction
> is optimized by br.
> 
Yes, sounds resonable.
> [...]
>> +int arch_prepare_optimized_kprobe(struct optimized_kprobe *op, struct kprobe *orig)
>> +{
>> +	kprobe_opcode_t *code;
>> +	u32 insn;
>> +	int ret, i;
>> +	void *addrs[TMPL_END_IDX];
>> +	void *addr;
>> +
>> +	code = get_optinsn_slot();
>> +	if (!code)
>> +		return -ENOMEM;
>> +
>> +	if (!is_offset_in_range((unsigned long)code,
>> +				(unsigned long)orig->addr + 8))
>> +		goto error;
>> +
>> +	if (!is_offset_in_range((unsigned long)code + TMPL_CALL_BACK,
>> +				(unsigned long)optimized_callback))
>> +		goto error;
>> +
>> +	if (!is_offset_in_range((unsigned long)&code[TMPL_RESTORE_END],
>> +				(unsigned long)op->kp.addr + 4))
>> +		goto error;
>> +
>> +	/* Setup template */
>> +	for (i = 0; i < TMPL_END_IDX; i++)
>> +		addrs[i] = code + i;
>> +
>> +	ret = aarch64_insn_patch_text(addrs, optprobe_template_entry,
>> +				      TMPL_END_IDX);
> 
> You should use aarch64_insn_patch_text_nosync() here (and all the
> aarch64_insn_patch_text() in this function too), because the insn
> buffer must not executed until the probe point is optimized.
> 
aarch64_insn_patch_text() could patch multi instructions to code[] each 
time and aarch64_insn_patch_text_nosync() could only patch one 
instruction each time, so maybe aarch64_insn_patch_text() is better here.

I'll replace other aarch64_insn_patch_text() in this function.

Thanks,
Qi

>> +	if (ret < 0)
>> +		goto error;
>> +
>> +	/* Set probe information */
>> +	addr = code + TMPL_VAL_IDX;
>> +	insn =  (unsigned long long)op & 0xffffffff;
>> +	aarch64_insn_patch_text(&addr, &insn, 1);
>> +
>> +	addr = addr + 4;
>> +	insn = ((unsigned long long)op & GENMASK_ULL(63, 32)) >> 32;
>> +	aarch64_insn_patch_text(&addr, &insn, 1);
>> +
>> +	addr = code + TMPL_CALL_BACK;
>> +	insn =  aarch64_insn_gen_branch_imm((unsigned long)addr,
>> +				(unsigned long)optimized_callback,
>> +				AARCH64_INSN_BRANCH_LINK);
>> +	aarch64_insn_patch_text(&addr, &insn, 1);
>> +
>> +	/* The original probed instruction */
>> +	addr = code + TMPL_RESTORE_ORIGN_INSN;
>> +	insn =  orig->opcode;
>> +	aarch64_insn_patch_text(&addr, &insn, 1);
>> +
>> +	/* Jump back to next instruction */
>> +	addr = code + TMPL_RESTORE_END;
>> +	insn = aarch64_insn_gen_branch_imm(
>> +				(unsigned long)(&code[TMPL_RESTORE_END]),
>> +				(unsigned long)(op->kp.addr) + 4,
>> +				AARCH64_INSN_BRANCH_NOLINK);
>> +	aarch64_insn_patch_text(&addr, &insn, 1);
>> +
>> +	flush_icache_range((unsigned long)code,
>> +			   (unsigned long)(&code[TMPL_END_IDX]));
>> +	/* Set op->optinsn.insn means prepared. */
>> +	op->optinsn.insn = code;
>> +
>> +	return 0;
> 
> Thank you,
> 
> 
>