Linux 6.2's Call Depth Tracking Helps Recover Lost Performance On Intel Skylake CPUs

When the Retbleed security vulnerability was introduced earlier this year mitigating it for Intel Skylake and Skylake-derived CPU cores required imposing Indirect Branch Restricted Speculation (IBRS) that further tanked the out-of-the-box performance for these aging Intel CPUs. But being introduced now with Linux 6.2 is a new mitigation technique named Call Depth Tracking that is helping recover some of that lost performance and in turn extending the usefulness of the Skylake-derived processors still in service.