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

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  Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16 Arnd Bergmann <arnd@arndb.de> - 2016-04-16 21:50 +0200
    Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16 Pan Xinhui <xinhui@linux.vnet.ibm.com> - 2016-04-18 12:20 +0200

#1380600 — Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16

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@linux.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()

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.

	Arnd

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#1381459

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@linux.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
> 

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