Re: [PATCH v3] arm: remove !CPU_V6 and !GENERIC_ATOMIC64 build dependenciesfor XEN

[Chen Gang F T]

On 01/10/2014 02:42 AM, Will Deacon wrote:
> On Thu, Jan 09, 2014 at 12:47:24PM +0000, Stefano Stabellini wrote:
>> On Thu, 9 Jan 2014, Arnd Bergmann wrote:
>>> On Thursday 09 January 2014, Will Deacon wrote:
>>>> On Wed, Jan 08, 2014 at 06:00:23PM +0000, Stefano Stabellini wrote:
>>>>> Remove !GENERIC_ATOMIC64 build dependency:
>>>>> - rename atomic64_xchg to armv7_atomic64_xchg and define it even ifdef
>>>>>   GENERIC_ATOMIC64;
>>>>> - call armv7_atomic64_xchg directly from xen/events.h.
>>>>>
>>>>> Remove !CPU_V6 build dependency:
>>>>> - introduce __cmpxchg8 and __cmpxchg16, compiled even ifdef
>>>>>   CONFIG_CPU_V6;
>>>>> - implement sync_cmpxchg using __cmpxchg8 and __cmpxchg16.
>>>>>
>>>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>>>>> CC: arnd@xxxxxxxx
>>>>> CC: linux@xxxxxxxxxxxxxxxx
>>>>> CC: will.deacon@xxxxxxx
>>>>> CC: gang.chen@xxxxxxxxxxx
>>>>> CC: catalin.marinas@xxxxxxx
>>>>> CC: jaccon.bastiaansen@xxxxxxxxx
>>>>> CC: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>>>> CC: linux-kernel@xxxxxxxxxxxxxxx
>>>>>
>>>>
>>>> I'm confused here. It looks like you want to call armv7 code in a v6 kernel.
>>>> What am I missing?
>>>
>>> This is about being able to build a kernel that runs on ARMv6 and ARMv7
>>> and also includes Xen. Because of obvious hardware limitations, Xen
>>> will only run on v7, but currently you cannot even build it once you
>>> enable (pre-v6K) ARMv6 support, since the combined v6+v7 kernel can't
>>> do atomic accesses in a generic way on non-32bit variables.
>>
>> Yep, that's right.
> 
> Ok, thanks for the explanation. Looking at the patch, I wonder whether it's
> not cleaner just to implement xchg code separately for Xen? The Linux code
> isn't always sufficient (due to the GENERIC_ATOMIC64 stuff) and most of the
> churn coming out of this patch is an attempt to provide some small code
> reuse at the cost of code readability.
> 
> What do others think?
> 

What Will said sounds reasonable to me.


Thanks.
-- 
Chen Gang
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