You probably heard of it: there's a new CPU vulnerability in town, affecting various CPUs, both Intel and AMD (and likely even non-x86 architectures: it's a pretty "wide" problem). So here is the software mitigation for it, directly added inside Xen hypervisor.
RETbleed is a speculative execution attack on x86-64 processors, including some recent Intel and AMD chips. You can read the original announcement from Computer Security Group at this address: https://comsec.ethz.ch/research/microarch/retbleed/
Despite sounding similar to previous speculative execution attacks targeting indirect jumps (the whole Spectre family), RETbleed exploits return instructions. And it's a big deal, since it defeats the existing Spectre-like mitigations in place. Not convinced? See this demo:
In terms of affected hardware, here is a list of tested CPUs against it:
Due to this hardware security flaw, an attacker might be able to infer the contents of arbitrary host memory, including memory assigned to other guests.