To: Jes Sorensen <jes@sgi.com>
Cc: "Bryan O'Sullivan" <bos@pathscale.com>, Andi Kleen <ak@suse.de>,
       linux-kernel@vger.kernel.org, discuss@x86-64.org
Subject: Re: Why is wmb() a no-op on x86_64?
References: <1137601417.4757.38.camel@serpentine.pathscale.com>
	<200601181729.36423.ak@suse.de>
	<1137603169.4757.50.camel@serpentine.pathscale.com>
	<yq04q41qxcw.fsf@jaguar.mkp.net>
From: Roland Dreier <rdreier@cisco.com>
Date: Wed, 18 Jan 2006 12:07:23 -0800
In-Reply-To: <yq04q41qxcw.fsf@jaguar.mkp.net> (Jes Sorensen's message of "18
 Jan 2006 12:06:39 -0500")
Message-ID: <ada4q41wb9g.fsf@cisco.com>
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    Bryan> On x86_64, we fiddle with the MTRRs to enable write
    Bryan> combining, which makes a huge difference to performance.
    Bryan> It's not clear to me what we should even do on other
    Bryan> architectures, since the only generic entry point that even
    Bryan> exposes write combining is pci_mmap_page_range, which is
    Bryan> for PCI mmap through userspace, and half the arches I've
    Bryan> looked at ignore its write_combine parameter.

    Jes> A job for mmiowb() perhaps?

I don't think the semantics of mmiowb() do what is desired here.
mmiowb() is all about ordering writes between separate CPUs, and the
issue at hand here is that write-combining buffers might reorder
writes from a single CPU.

 - R.
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