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

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Message-ID: <alpine.DEB.2.02.1401091247000.21510@kaball.uk.xensource.com>
Date:	Thu, 9 Jan 2014 12:47:24 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <will.deacon@....com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"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, 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.
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