Re: [Qemu-devel] [PATCH] Inter-VM shared memory PCI device

[malc <av1474@comtv.ru>]

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On Thu, 11 Mar 2010, Nick Piggin wrote:

> On Thu, Mar 11, 2010 at 03:10:47AM +0000, Jamie Lokier wrote:
> > Paul Brook wrote:
> > > > > In a cross environment that becomes extremely hairy.  For example the x86
> > > > > architecture effectively has an implicit write barrier before every
> > > > > store, and an implicit read barrier before every load.
> > > > 
> > > > Btw, x86 doesn't have any implicit barriers due to ordinary loads.
> > > > Only stores and atomics have implicit barriers, afaik.
> > > 
> > > As of March 2009[1] Intel guarantees that memory reads occur in
> > > order (they may only be reordered relative to writes). It appears
> > > AMD do not provide this guarantee, which could be an interesting
> > > problem for heterogeneous migration..
> > 
> > (Summary: At least on AMD64, it does too, for normal accesses to
> > naturally aligned addresses in write-back cacheable memory.)
> > 
> > Oh, that's interesting.  Way back when I guess we knew writes were in
> > order and it wasn't explicit that reads were, hence smp_rmb() using a
> > locked atomic.
> > 
> > Here is a post by Nick Piggin from 2007 with links to Intel _and_ AMD
> > documents asserting that reads to cacheable memory are in program order:
> > 
> >     http://lkml.org/lkml/2007/9/28/212
> >     Subject: [patch] x86: improved memory barrier implementation
> > 
> > Links to documents:
> > 
> >     http://developer.intel.com/products/processor/manuals/318147.pdf
> >     http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/24593.pdf
> > 
> > The Intel link doesn't work any more, but the AMD one does.
> 
> It might have been merged into their development manual now.

It was (http://www.intel.com/products/processor/manuals/):

Intel╝ 64 Architecture Memory Ordering White Paper

This document has been merged into Volume 3A of Intel 64 and IA-32 
Architectures Software Developer's Manual.

[..snip..]

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