I mean it depends on when you need to do it, right? If there are vulnerabilities that lead to private kernel data leaking to userspace through the L1D, you’re talking about needing to wipe out your data cache on every system call, which might need to happen millions of times per second.

Also, context switches can be very frequent in some designs. For example, in micro kernel systems you often have ping-ponging with processes communicating with servers via RPC. Wiping out your whole L1D every time that happens could be pretty unpleasant.