Detailing SELinux,cgroups pros and cons vs Hypervisor. (#52)

Add details about SELinux cgroups vs hypervisor

Signed-off-by: igor stoppa <[email protected]>
Co-authored-by: Paul Albertella <[email protected]>

docs/Interference_Scenarios_for_an_ARM64_Linux_System.md

@@ -472,6 +472,16 @@ This is perfectly fine from the perspective of containing user-space, however, s
 frequency execution of a large amount of code which can cause either direct or indirect interference.
 And such interference is not always detectable, depending on which component it might affect.
 
+These features are usually enabled in a mixed criticality scenario, when attempting to contain interference.
+The choice of enabling them, despite the associated risk, might be driven by overall considerations about choosing the lesser evil.
+Of course one could attemtp to qualify them, but then it is necessary to consider the fact that in reality it is necessary to qualify them together with the user-space-provided policies they will enact.
+Without being configured by user-space, neither SELinux nor cgroups are of any particular use.
+
+An alternative - possibly more costly - path could be to instead isolate more safety-relevant loads from non-safety-relevant ones, introducing a second virtual machine, with a hypervisor underneath.
+The caveat is that now the hypervisor can be a source of interference. And it is also necessary to have HW capable to support an EL2.
+It can be an interesting alternative, though, if using a Type1 hypervisor (like Xen), because it is relatively simple in comparison to qualifying the Linux code.
+
+
 ## Sources of Interference
 It is useful to model the most probable causes for spatial interference, even if not exhaustively.