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

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Message-Id: <200608030739.13334.ak@suse.de>
Date:	Thu, 3 Aug 2006 07:39:13 +0200
From:	Andi Kleen <ak@...e.de>
To:	Christoph Lameter <clameter@....com>
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 Thursday 03 August 2006 07:32, Christoph Lameter wrote:
> On Thu, 3 Aug 2006, Andi Kleen wrote:
> 
> > > As far as I can tell from this conversation there are special "Xen" 
> > > drivers that need this not the rest of the system.
> > 
> > Yes, but in general when a driver that runs on multiple architectures
> > (including IA64 btw) needs something architecture specific we usually
> > add it to asm, not add ifdefs.
> 
> 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.

> 
> > > What possible use could there be to someone else?
> > 
> > e.g. for other hypervisors or possibly for special hardware access
> > (e.g. I could imagine it being used for some kind of cluster interconnect)
> > I remember Alan was using a similar hack in his EDAC drivers because
> > it was the only way to clear ECC errors. 
> 
> 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.
 
> > Just adding a single line #include for a wrapper asm-generic surely isn't
> > a undue burden for the other architectures, and it will save some
> > mess in the Xen drivers.
> 
> 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.
-Andi

 
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