linux-kernel - Re: [PATCH v5 2/7] arm64: introduce interfaces to hotpatch kernel and module code

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Message-ID: <5297C61F.8000801@linaro.org>
Date:	Fri, 29 Nov 2013 07:39:27 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Jiang Liu <liuj97@...il.com>, Will Deacon <will.deacon@....com>
CC:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 2/7] arm64: introduce interfaces to hotpatch kernel
 and module code

On 11/04/2013 12:55 AM, Jiang Liu wrote:
> On 10/30/2013 08:12 AM, Will Deacon wrote:
>> Hi Jinag Liu,
>>
>>> +static __always_inline u32 aarch64_insn_read(void *addr)
>>> +{
>>> +	return le32_to_cpu(*(u32 *)addr);
>>> +}
>>>
>>> +static __always_inline void aarch64_insn_write(void *addr, u32 insn)
>>> +{
>>> +	*(u32 *)addr = cpu_to_le32(insn);
>>> +}
>>
>> I wouldn't bother with these helpers. You should probably be using
>> probe_kernel_address or similar, then doing the endianness swabbing on the
>> return value in-line.
> How about keeping and refining aarch64_insn_read/write interfaces
> by using probe_kernel_address()? I think they may be used in other
> places when supporting big endian ARM64 kernel.

I prefer it (using probe_kernel_read/write) for my ftrace patch.
I would be able to replace some portion of my own function (ftrace_modify_code)
to aarch64_insn_patch_text_nosync().

See my comment here:
http://lists.infradead.org/pipermail/linux-arm-kernel/2013-October/207001.html
([PATCH 2/6])

Current implementation assumes stop_machine (via arch_ftrace_update_code()
in generic ftrace), and, given the discussion btw you and Will, I wonder
that it might be relaxed because ftrace on arm64 modifies only a single branch
or nop instruction at any time.

-Takahiro AKASHI
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