Linux 7.4 Adapts AMD Zen 6 For Less Costly CPU Security Mitigation

The x86/bugs pull request was merged this week for the Linux 7.4 kernel. Most notable here is adjusting the CPU security mitigations for AMD Zen 6 processors. With AMD Zen 6 processors there is a new BTB CTX isolation feature for the Branch Target Buffer to better isolate it between different contexts like user/kernel and guest/host contexts. In turn the SafeRET Speculative Return Stack Overflow (SRSO) mitigation can be safely disabled for Zen 6 processors. The x86/bugs pull request does that with tapping the BTB CTX isolation feature in place of the "expensive" SRSO SafeRET mitigation. Thus the stock Linux 7.4 kernel takes care of that more optimal behavior of using the new CPU capabilities. We'll see if this patch also gets back-ported to prior Linux kernel stable series in the coming days. It will also be interesting to run some AMD Zen 6 CPU comparison benchmarks of the mitigation impact on system performance. "- AMD Zen6's BTB has per-context tagging so there's no need for the expensive SRSO SafeRET mitigation anymore; add glue to detect and handle that - Fix Spectre v2 mitigation reporting for RETPOLINE=n builds" That's it for the x86/bugs pull request in Linux 7.4.