However, I believe Solaris uses separate address spaces for user and kernel on SPARC. So that should avoid meltdown at least.

Also, it is unclear whether it allows speculation beyond privilege boundaries and it is also unclear whether ordering semantics for memory are different enough to mitigate somewhat.

I wish I still had access to modern sparc hardware to find out.

While it’s best to assume everything is vulnerable unless proven otherwise I’d really like to see a thorough evaluation.