Re: [PATCH 3/3] arm64/kprobe: Optimize the performance of patching single-step slot

[liaochang (A)]

在 2022/9/23 20:39, Mark Rutland 写道:
> On Fri, Sep 23, 2022 at 04:46:58PM +0800, Liao Chang wrote:
>> Single-step slot would not be used until kprobe is enabled, that means
>> no race condition occurs on it under SMP, hence it is safe to pacth ss
>> slot without stopping machine.
> 
> I think this is correct, but this depends on a couple of subtleties,
> importantly:
> 
> * That the I-cache maintenance for these instructions is complete *before* the
>   kprobe BRK is written (and aarch64_insn_patch_text_nosync() ensures this, but
>   just omits causing a Context-Synchronization-Event on all CPUS).

So in order to guarantee the I-cache maintenance is observed on all CPUS,
it needs to be followed by a explicit Context-Synchronization-Event, perhaps
it is better to place ISB before kprobe BRK is written.

> 
> * That the kprobe BRK results in an exception (and consequently a
>   Context-Synchronoization-Event), which ensures that the CPU will fetch the
>   single-step slot instructions *after* this, ensuring that the new
>   instructions are used.

Yes, because of single-step slot is installed int the BRK execption handler,
so it is not necessary to generate Context-Synchronization-Event via ISB mentioned above...

Thanks.

> 
> It would be good if we could call that out explicitly.
> 
> Thanks,
> Mark.
> 
>> Since I and D caches are coherent within single-step slot from
>> aarch64_insn_patch_text_nosync(), hence no need to do it again via
>> flush_icache_range().
>>
>> Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>> Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
>> ---
>>  arch/arm64/kernel/probes/kprobes.c | 7 ++-----
>>  1 file changed, 2 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
>> index d1d182320245..29b98bc12833 100644
>> --- a/arch/arm64/kernel/probes/kprobes.c
>> +++ b/arch/arm64/kernel/probes/kprobes.c
>> @@ -44,13 +44,10 @@ post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
>>  static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
>>  {
>>  	kprobe_opcode_t *addr = p->ainsn.api.insn;
>> -	void *addrs[] = {addr, addr + 1};
>> -	u32 insns[] = {p->opcode, BRK64_OPCODE_KPROBES_SS};
>>  
>>  	/* prepare insn slot */
>> -	aarch64_insn_patch_text(addrs, insns, 2);
>> -
>> -	flush_icache_range((uintptr_t)addr, (uintptr_t)(addr + MAX_INSN_SIZE));
>> +	aarch64_insn_patch_text_nosync(addr, p->opcode);
>> +	aarch64_insn_patch_text_nosync(addr + 1, BRK64_OPCODE_KPROBES_SS);
>>  
>>  	/*
>>  	 * Needs restoring of return address after stepping xol.
>> -- 
>> 2.17.1
>>
> 
> .

-- 
BR,
Liao, Chang