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

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Date:	Thu, 9 Jan 2014 18:42:51 +0000
From:	Will Deacon <will.deacon@....com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	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, 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?

Will
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