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[Xen-devel] Re: [patch 2/8] Implement always-locked bit ops, for memory

To: Andi Kleen <ak@xxxxxxx>
Subject: [Xen-devel] Re: [patch 2/8] Implement always-locked bit ops, for memory shared with an SMP hypervisor.
From: Christoph Lameter <clameter@xxxxxxx>
Date: Wed, 2 Aug 2006 22:54:17 -0700 (PDT)
Cc: akpm@xxxxxxxx, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx
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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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