[PATCH] memorder: Add a brief description of the use of multicopy atomicity

[Junchang Wang <junchangwang@gmail.com>]

Subsection Multicopy Atomicity is ahead of Subsection Hardware Specifics where
implementation detail of multicopy atomicity in mainstream architectures is
presented. So we add (1) a brief description of the use of multicopy atomicity
in representative architectures and (2) a reference to Table Summary of Memory
Ordering, in subsection Multicopy Atomicity.

The description is borrowed from a nice discussion with Paul, which can be found
at https://www.spinics.net/lists/perfbook/msg01952.html.

Signed-off-by: Junchang Wang <junchangwang@gmail.com>
---
 memorder/memorder.tex | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/memorder/memorder.tex b/memorder/memorder.tex
index fba49b7..69ce196 100644
--- a/memorder/memorder.tex
+++ b/memorder/memorder.tex
@@ -1901,7 +1901,11 @@ Most CPU vendors interested in providing multicopy atomicity have therefore
 instead provided the slightly weaker
 \emph{other-multicopy atomicity}~\cite[Section B2.3]{ARMv8A:2017},
 which excludes the CPU doing a given store from the requirement that all
-CPUs agree on the order of all stores.
+CPUs agree on the order of all stores.\footnote{
+	As of late 2018, ARMv8 and x86 provide other-multicopy atomicity,
+	IBM mainframe provides fully multicopy atomicity, and PPC does
+	not provide multicopy atomicity at all. More detail is shown in
+	Figure~\ref{tab:memorder:Summary of Memory Ordering}.}
 This means that if only a subset of CPUs are doing stores, the
 other CPUs will agree on the order of stores, hence the ``other''
 in ``other-multicopy atomicity''.
-- 
2.7.4