linux-kernel - Re: [patch 2/8] Implement always-locked bit ops, for memory shared with an SMP hypervisor.

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Date:	Wed, 2 Aug 2006 22:54:17 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andi Kleen <ak@...e.de>
cc:	virtualization@...ts.osdl.org,
	Jeremy Fitzhardinge <jeremy@...p.org>, akpm@...l.org,
	xen-devel@...ts.xensource.com, Chris Wright <chrisw@...s-sol.org>,
	Ian Pratt <ian.pratt@...source.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/8] Implement always-locked bit ops, for memory shared
 with an SMP hypervisor.

On Thu, 3 Aug 2006, Andi Kleen wrote:

> > I still wonder why you are so focused on ifdefs. Why would we need those?
> 
> Because the Xen drivers will run on a couple of architectures, including
> IA64 and PPC.
> 
> If IA64 or PPC didn't implement at least wrappers for the sync ops
> then they would all need special ifdefs to handle this.

No they would just need to do an #include <xen-bitops.h>

> > Maybe the best thing would be to have proper atomic ops in UP mode on 
> > i386? The current way of just dropping the lock bit is the source of the 
> > troubles.
> 
> It's a huge performance difference.

I understand but why dont we use regular ops explicitly 
instead of hacking the atomic ops. Then we would not have unhack them now.

> > Just adding a single line #include <asm/xen-bitops.h> to drivers that need 
> > this functionality is not an undue burden for the drivers that support 
> > Xen. They have to use special _xxx bitops anyways.
> 
> Ok it could be put into a separate file (although with a neutral name)
> 
> But you would still need to add that to IA64, PPC etc. too, so it 
> would only avoid adding a single to the other architectures.

Could we not just add one fallback definition to asm-generic?

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