gem5: summarize the differences between the main cores

README.adoc

@@ -12304,6 +12304,23 @@ In an Ubuntu 18.04 host where `python` is `python2` by default, build with Pytho
 
 Python 3 is then automatically used when running if you use that build.
 
+=== gem5 CPU types
+
+gem5 has a few in tree CPU models for different purposes, here is an overview of the most interesting ones:
+
+* `BaseSimpleCPU` descendants. Have no CPU pipeline.
+** `AtomicSimpleCPU`: the default one. Memory accesses happen instantaneously. The fastest simulation except for KVM, but not realistic at all. Useful to <<gem5-restore-checkpoint-with-a-different-cpu>>.
+** `TimingSimpleCPU: memory accesses are realistic, but the CPU has no pipeline. The simulation is faster than detailed models, but slower than `AtomicSimpleCPU`. TODO: application?
+* `MinorCPU`: in-order core. Its 4 stage pipeline is described at the "MinorCPU" section of <<gem5-arm-rsk>>.
+** `HPI`: derived from `MinorCPU` simply by parametrization. According to <<gem5-arm-rsk>>: "The HPI CPU timing model is tuned to be representative of a modern in-order Armv8-A implementation."
+* `DerivO3CPU`: out-of-order core
+
+==== gem5 ARM RSK
+
+https://github.com/arm-university/arm-gem5-rsk/blob/aa3b51b175a0f3b6e75c9c856092ae0c8f2a7cdc/gem5_rsk.pdf
+
+Dated 2017, it contains a good overview of gem5 CPUs.
+
 === gem5 ARM platforms
 
 The gem5 platform is selectable with the `--machine` option, which is named after the analogous QEMU `-machine` option, and which sets the `--machine-type`.