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

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Date:	Thu, 16 Jan 2014 16:27:55 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Will Deacon <will.deacon@....com>
CC:	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>,
	"jaccon.bastiaansen@...il.com" <jaccon.bastiaansen@...il.com>
Subject: Re: [PATCH v3] arm: remove !CPU_V6 and !GENERIC_ATOMIC64 build
 dependencies for XEN

On Thu, 9 Jan 2014, 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?

I am OK with that, in fact my first version of the patch did just that:

http://marc.info/?l=linux-arm-kernel&m=138436406724990&w=2

Is that what you had in mind?
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