linux-kernel - Re: [PATCH v3] arm: remove !CPU_V6 and !GENERIC

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Message-ID: <CAGzjT4dQo22uErD4GuiifoZXjSmM9XHQHn+gKqZDJDqfJTzpmw@mail.gmail.com>
Date:	Mon, 13 Jan 2014 09:12:07 +0100
From:	Jaccon Bastiaansen <jaccon.bastiaansen@...il.com>
To:	Chen Gang F T <chen.gang.flying.transformer@...il.com>
Cc:	Will Deacon <will.deacon@....com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"gang.chen@...anux.com" <gang.chen@...anux.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] arm: remove !CPU_V6 and !GENERIC_ATOMIC64 build
 dependencies for XEN

2014/1/10 Chen Gang F T <chen.gang.flying.transformer@...il.com>:
> 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@...citrix.com>
>>>>>> CC: arnd@...db.de
>>>>>> CC: linux@....linux.org.uk
>>>>>> CC: will.deacon@....com
>>>>>> CC: gang.chen@...anux.com
>>>>>> CC: catalin.marinas@....com
>>>>>> CC: jaccon.bastiaansen@...il.com
>>>>>> CC: linux-arm-kernel@...ts.infradead.org
>>>>>> CC: linux-kernel@...r.kernel.org
>>>>>>
>>>>>
>>>>> 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

I agree with Will,

Regards,
  Jaccon
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