[Xen-devel] Re: [RFC PATCH 02/33] Add sync bitops

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 18 Jul 2006, at 10:56, Arjan van de Ven wrote:

> > plain text document attachment (synch-ops)
> > 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.
> Hi,
>
> this sounds really like the wrong approach; you know you're compiling
> for xen, so why not just make set_bit() and the others use the lock'd
> instructions at compile time?
Then all users of bitops would unnecessarily pay the price when running 
in a single-CPU guest. Only a few of our bitops callers absolutely 
require the lock prefix in all cases.
  -- Keir


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