Interesting! Some game launcher tools use GameMode which disables this mitigation by default while active: Allow user to disable the Linux kernel split lock mitigation by HenrikHolst · Pull Request #446 · FeralInteractive/gamemode · GitHub
GameMode is available as a Fedora package. May be more convenient to use than setting this manually.
Would this be anything to do with emulating NTSYNC?
And once the steam, etc. supports kernel has the ntsync it stopps being a problem?
Not really. It is much more low-level than that. In short, a variable is updated in a way that no other process or thread can go in between and also modify the same memory position. If the variable is mis-alligned, for example address 0x123000FF and it is 4 bytes long, the cpu must firt fetch 1 byte from 0x123000FF and the remaining 3 from 0x12300100. If this must be done atomically, the whole computer is basically frozen for the time it takes. That is the split lock access. If the variable is properly alligned the cpu can fetch all 4 bytes in one operation and the other cpu processors can just keep on running normally. It is actually more complicated than that.
It looks like this was a feature added around the Tiger Lake generation, so that’s why it doesn’t affect my i7-10700F computer.