Following Retbleed, The Combined CPU Security Mitigation Impact For AMD Zen 2 / Ryzen 9 3950X
The default out-of-the-box in I/O workloads shows some impact with the Retbleed untrained return thunks as well as other mitigations like Spectre V2 Retpolines, conditional STIBPs, etc. But if opting for the IBPB Retbleed protection mode is where the performance really nosedives quite quickly. The test runs involving retbleed=ibpb saw severely degraded performance with that more intense Retbleed protection.
The Retbleed IBPB mitigation on the Ryzen 9 3950X also saw significantly degraded performance for the WireGuard Linux kernel networking stress test.
For some OpenJDK Java tests, they actually enjoyed better performance with SMT disabled. The Retbleed IBPB mitigation did have some impact here for the SMT-enabled setups while the mitigations=off versus stock kernel configuration was quite minimally impacted here.