[kvm-unit-tests PATCH 3/3] README: Add a guide of how to run tests with litmus7

[Nikos Nikoleris <nikos.nikoleris@arm.com>]

litmus7 (http://diy.inria.fr/doc/litmus.html) is a tool that takes as
an input a litmus test specification and generates a program that can
execute on hardware. Using kvm-unit-tests, litmus7 can generate tests
that run on KVM. This change adds some documentation to introduce this
functionality along with a basic example of how to build and run such
tests.

Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
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+# Memory model litmus tests as kvm-unit-tests
+
+In this guide, we explain how to use kvm-unit-tests in combination
+with litmus7 to generate and run litmus tests as tiny operating
+systems.
+
+## Background
+
+### Memory model litmus tests
+
+A litmus test is a small program designed to exercise a certain
+behavior. Traditionally, litmus tests have been used to demonstrate
+and exercise the memory model of parallel computing systems. Litmus
+tests are often described in assembly or pseudo-assembly code and
+require tools like [litmus7](http://diy.inria.fr/doc/litmus.html) to
+genererate executable programs that we can then run on real hardware.
+
+### Why kvm-unit-tests
+
+litmus7 uses kvm-unit-tests to encapsulate a litmus tests and generate
+executables in the form of tiny operating systems. Inside these tiny
+operating systems, the litmus tests can control parameters of the
+execution that a user space application cannot. For example, control
+virtual address translation and handle exceptions.
+
+## Building and running litmus kvm-unit-tests
+
+litmus7 is a tool that given a litmus test will generate C source
+code. The generated C source code is compiled and linked with
+kvm-unit-tests to generate the binary.
+
+## Prerequisites
+
+litmus7 is part of the herdtools7 toolsuite. See
+https://github.com/herd/herdtools7/blob/master/INSTALL.md for
+instructions of how to build and install herdtools7 from source.
+
+In addition to herdtools7, this guide assumes that you have a copy of
+kvm-unit-tests and you have already configured and built the included
+tests.
+
+## Building and running a litmus test
+
+In this guide, we use `MP` (Message Passing), a popular litmus test
+which demonstrates the communication pattern between a sender (P0) and
+a receiver (P1_ process of a message `x`. There is also variable `y`
+which is a flag, the sender sets it after setting the message `x` and
+the receiver reads it before reading the message `x`. The test also
+specifies a condition after the execution of the test which is
+validated when P1 reads the initial value of message `x` (0) and the
+new value of flag `y` (1).
+
+```
+AArch64 MP
+"PodWW Rfe PodRR Fre"
+Generator=diyone7 (version 7.56)
+Prefetch=0:x=F,0:y=W,1:y=F,1:x=T
+Com=Rf Fr
+Orig=PodWW Rfe PodRR Fre
+{
+0:X1=x; 0:X3=y;
+1:X1=y; 1:X3=x;
+}
+ P0          | P1          ;
+ MOV W0,#1   | LDR W0,[X1] ;
+ STR W0,[X1] | LDR W2,[X3] ;
+ MOV W2,#1   |             ;
+ STR W2,[X3] |             ;
+exists (1:X0=1 /\ 1:X2=0)
+```
+
+Note: [diy7](http://diy.inria.fr/doc/gen.html) and
+[diyone7]](http://diy.inria.fr/doc/gen.html) from the herdtools7
+toolsuite can systematically generate litmus tests. For example to
+generate the MP litmus test:
+
+    diyone7 -arch AArch64 PodWW Rfe PodRR Fre -name MP
+
+Assuming the file `MP.litmus` contains the test and KUT_DIR is the top
+directory of kvm-unit-tests, we can use litmus7 to generate
+the C sources:
+
+    litmus7 -mach kvm-armv8.1 -variant precise -a 4 -o ${KUT_DIR}/litmus_tests MP.litmus
+
+To build the test:
+
+    cd litmus-tests && make && cd -
+
+This will build the test in the file `litmus-tests/MP.flat` which you
+can run like any other test:
+
+     ./arm/run litmus-tests/MP.flat -smp 4
+
+The test will print whether the condition of the test was observed
+(`Allowed`), stats about the observed outcomes and metadata (e.g.,
+hash).
+
+```
+Test MP Allowed
+Histogram (4 states)
+19865 :>1:X0=0; 1:X2=1;
+20885 *>1:X0=1; 1:X2=0;
+975348:>1:X0=0; 1:X2=0;
+983902:>1:X0=1; 1:X2=1;
+Ok
+
+Witnesses
+Positive: 20885, Negative: 1979115
+Condition exists (1:X0=1 /\ 1:X2=0) is validated
+Hash=211d5b298572012a0869d4ded6a40b7f
+Cycle=Rfe PodRR Fre PodWW
+Generator=diycross7 (version 7.54+01(dev))
+Com=Rf Fr
+Orig=PodWW Rfe PodRR Fre
+Observation MP Sometimes 20885 1979115
+Time MP 3.19
+```
+
+## References
+
+https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/expanding-memory-model-tools-system-level-architecture
+https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/running-litmus-tests-on-hardware-litmus7
+http://diy.inria.fr/doc/litmus.html
+https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/generate-litmus-tests-automatically-diy7-tool
+http://diy.inria.fr/doc/gen.html
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