Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining

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

On Tue, 2006-02-28 at 20:33 +0100, Andi Kleen wrote:

> Anyways if MFENCE improved performance you're probably relying
> on some very specific artifact of the microarchitecture of your 
> CPU or Northbridge. I don't think it's a architecurally guaranteed
> feature.

I looked this up, and you appear to be wrong here.

Here's the appropriate quote from page 246 of the PDF of "AMD64
Architecture Programmer's Manual Volume 2: System Programming":

        http://www.amd.com/us-en/assets/content_type/DownloadableAssets/dwamd_24593.pdf

Section 7.4.1 specifically describes what happens to write buffers:

        [...] the processor completely empties the write buffer by
        writing the contents to memory as a result of performing any of
        the following operations:
        
        SFENCE Instruction
        Executing a store-fence (SFENCE) instruction forces all memory
        writes before the SFENCE (in program order) to be written into
        memory before memory writes that follow the SFENCE instruction.
        The memory-fence (MFENCE) instruction has a similar effect, but
        it forces the ordering of loads in addition to stores.
        [...]

So in fact SFENCE is the appropriate, architecturally guaranteed, thing
for us to be doing on x86_64.

With respect to Ben's contention that wmb() will suffice instead, that
isn't true, either, even on x86-class hardware.  The writes absolutely
travel over the HT bus in non-ascending order on AMD64 systems unless we
fence them, and we've verified this using a HT bus analyser.

	<b

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