linux-kernel - Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16

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Message-ID: <5714B4B7.2050005@linux.vnet.ibm.com>
Date:	Mon, 18 Apr 2016 18:19:35 +0800
From:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>, linuxppc-dev@...ts.ozlabs.org
CC:	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16



On 2016年04月17日 03:43, Arnd Bergmann wrote:
> On Wednesday 13 April 2016 19:15:17 Pan Xinhui wrote:
>> Hello Peter,
>>
>> On 2016年04月12日 22:30, Peter Zijlstra wrote:
>>> On Sun, Apr 10, 2016 at 10:17:28PM +0800, Pan Xinhui wrote:
>>>>
>>>> On 2016年04月08日 15:47, Peter Zijlstra wrote:
>>>>> On Fri, Apr 08, 2016 at 02:41:46PM +0800, Pan Xinhui wrote:
>>>>>> From: pan xinhui <xinhui.pan@...ux.vnet.ibm.com>
>>>>>>
>>>>>> Implement xchg{u8,u16}{local,relaxed}, and
>>>>>> cmpxchg{u8,u16}{,local,acquire,relaxed}.
>>>>>>
>>>>>> Atomic operation on 8-bit and 16-bit data type is supported from power7
>>>>>
>>>>> And yes I see nothing P7 specific here, this implementation is for
>>>>> everything PPC64 afaict, no?
>>>>>
>>>> Hello Peter,
>>>> 	No, it's not for every ppc. So yes, I need add #ifdef here. Thanks for pointing it out.
>>>> We might need a new config option and let it depend on POWER7/POWER8_CPU or even POWER9...
>>>
>>> Right, I'm not sure if PPC has alternatives, but you could of course
>>> runtime patch the code from emulated with 32bit ll/sc to native 8/16bit
>>> ll/sc if present on the current CPU if you have infrastructure for these
>>> things.
>>>
>> seems interesting. I have no idea about how to runtime patch the code. I will try to learn that.
>> If so, we need change {cmp}xchg into uninline functions?
> 
> I think you don't need to, see do_feature_fixups()/patch_feature_section()
> 
Hello, Arnd
	thanks for your tips :) I will take a look at them.
This time I will make generic functions for all ppc. But in future, We will runtime patch the code.

> Note that an #ifdef by itself has to worry about any combination of
> architectures, so in a kernel that has both POWER6 and POWER7 enabled,
> you cannot call the POWER7-only function.
> 
seems your are right.
While I think it's not a good idea to enable several cpu types. just select the minimum supported cpu in real world. :)

thanks
xinhui
> 	Arnd
>