It’s turned off and on by the sysctl machdep.isolated_user_pmap – and defaults to on for Intel CPUs. Buildworld tests show about a 4-5% performance hit, but that’s only one form of activity, measured, so there will surely be other effects.
Note that Spectre is not mitigated by this commit series, and as I understand it, cannot be realistically fixed in software.
Update: Matthew Dillon posted a summary to users@.
Update 2: He told us so.