IIUC on Linux you can enable CONFIG_SCHED_CORE/PR_SCHED_CORE to realize that idea. [1] Some caveats may apply...

edit: to be clear, I just learned about retbleed five minutes ago, so I'm definitely not promising core scheduling mitigates it. I do think core scheduling is a good idea in general.

[1] https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/c...

Is that even enough? Hyperthreading let some of the earlier spectre exploits work faster, but as long as there's some shared cached between two cores (L2 or L3), that's enough for timing variations to leak what's in that cache afaik. Maybe it's enough for this attack (unsure), but I don't think it solves speculative side-channel leaks in general.