Linux 6.3 To Squeeze A Bit More Performance Out Of AMD Zen 4 CPUs

For some initial Automatic IBRS testing this week, I carried out a tip/tip.git x86/cpu Git kernel build on an AMD Ryzen 9 7950X desktop for seeing how the performance compares to the current Linux 6.2 Git using the same kernel configuration. Booting to the new kernel with the code expected for Linux 6.3, indeed the Ryzen 9 7950X defaulted to the Automatic IBRS mitigation for Spectre V2 mitigation compared to the current Retpoline approach.

This AMD Ryzen 9 7950X, AMD Radeon RX 6800 XT, ASUS ROG CROSSHAIR X670E HERO, 2 x 16GB DDR5-6000 memory, and WD_BLACK SN850X 2TB NVMe SSD desktop indeed was showing better performance for Automatic IBRS enabled kernel for workloads with high kernel interactions / sensitive to Spectre V2 mitigations.

Workloads like Redis, PostgreSQL, DragonflyDB, and others were enjoying a few percent better performance or more in going the Automatic IBRS route compared to Retpolines used up to this point on Zen 4. Again, this will be very exciting for AMD EPYC 9004 series performance given more workloads there tend to be susceptible to mitigation impact. I should have out some AMD EPYC 9004 series benchmarks next week with the Automatic IBRS enabled across a wide range (hundreds) of workloads... My lone AMD EPYC Genoa server was sadly tied up with other benchmarks at the moment. Thus today's run is just a starter for getting a feel for the difference of Automatic IBRS given AMD hadn't published any benchmark numbers and no other public information that I have seen on Automatic IBRS expectations relative to Retpolines.

This Automatic IBRS enabling with the Linux kernel is looking good for helping squeeze a bit more performance out of Zen 4 processors for some workloads, but unfortunate that this wasn't already wired up for launch day... Especially given the release alignment of the upstream Linux kernel and distribution/vendor adoption that it may still be months before Ryzen 7000 / EPYC 9004 users find Automatic IBRS being used over Retpolines.

Beyond Automatic IBRS, Linux 6.3 may be even more exciting for AMD Linux customers too if the AMD P-State EPP driver code manages to get wrapped up in time among other patches that still have time to settle in the next few weeks.