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

Andi Kleen
2006-05-09T23:04:39Z

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