Re: Why is wmb() a no-op on x86_64?

["Bryan O'Sullivan" <bos@pathscale.com>]

On Wed, 2006-01-18 at 17:29 +0100, Andi Kleen wrote:

> Actually it is a compiler optimizer barrier, not a no-op.

Sorry, braino.

> Hmm, I suppose one could add a wc_wmb() or somesuch, but WC 
> is currently deeply architecture specific so I'm not sure
> how you can even use it portably.
> 
> Why do you need the barrier?

On x86_64, we fiddle with the MTRRs to enable write combining, which
makes a huge difference to performance.  It's not clear to me what we
should even do on other architectures, since the only generic entry
point that even exposes write combining is pci_mmap_page_range, which is
for PCI mmap through userspace, and half the arches I've looked at
ignore its write_combine parameter.

	<b