[Xen-devel] Re: [RFC PATCH 05/35] Add sync bitops

[Andi Kleen <ak@xxxxxxx>]

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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