jlgreathouse / amd_ibs_toolkit Goto Github PK

While this repository doesn't mention SpecLockMap by name, FAM17H_MSR_WA_1 is MSRC001_1020[54]:

We found that MSRC001_1020[54] disables SpecLockMap (some advanced form of lock speculation which may roll back a number of post-lock instructions but doesn't correctly roll back performance counters) in rr-debugger/rr#2034 (comment).

Latest rr (from git) now contains some detection logic for SpecLockMap (execute a single atomic, check if r0825:u aka SpecLockMapCommit counted it), so you could use for testing (sadly I don't think there's a standalone version).

What we also know is that Zen 2 is just as affected by SpecLockMap's missing counter rollback as Zen 1 or Zen 1+, which leads me to believe ce475e6 should've added Zen 2 to this list:

Furthermore, only on Zen 2 will setting the MSR bit stick, due to the kernel using MSRC001_1020[10] as SSBD on earlier Zen (whereas Zen 2 got an architectural MSR for speculation control, and the kernel toggles that instead).

https://github.com/mozilla/rr/wiki/Zen contains more information on how to check if the workaround sticks on a given machine and what you can do to mitigate that - I suspect the tracepoint-based (kernel module) workaround by @glandium is the only one that would be readily applicable for you, since you're already operating within a kernel module (though currently you don't require tracepoints so that would be a new requirement for using the IBS Toolkit).