From: Andi Kleen <ak@suse.de>
To: virtualization@lists.osdl.org
Subject: Re: [RFC PATCH 05/35] Add sync bitops
Date: Wed, 10 May 2006 01:04:34 +0200
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Cc: Christoph Lameter <clameter@sgi.com>, Chris Wright <chrisw@sous-sol.org>,
       xen-devel@lists.xensource.com, linux-kernel@vger.kernel.org,
       Ian Pratt <ian.pratt@xensource.com>
References: <20060509084945.373541000@sous-sol.org> <20060509085149.024456000@sous-sol.org> <Pine.LNX.4.64.0605091555540.30338@schroedinger.engr.sgi.com>
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On Wednesday 10 May 2006 00:56, Christoph Lameter wrote:
> On Tue, 9 May 2006, Chris Wright wrote:
> 
> > Add "always lock'd" implementations of set_bit, clear_bit and
> > change_bit and the corresponding test_and_ functions.  Also add
> > "always lock'd" implementation of cmpxchg.  These give guaranteed
> > strong synchronisation and are required for non-SMP kernels running on
> > an SMP hypervisor.
> 
> Could you explain why this is done and what is exactly meant with "always 
> looked"? Wh the performance impact?

When UP guest runs on SMP hypervisor they still need the LOCK prefix
to talk to the hypervisor through shared memory in a smp safe way.

Normally UP kernels don't use any LOCK prefixes.

I suggested to refactor the bitops this way earlier for this.

-Andi

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