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

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.


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