I was (more) of a Solaris/x86 kernel guy on that particular level and know the x86 kernel did not use task gates for doors (or any context switching other than the double fault handler). Linux did taskswitch via task gates on x86 till 2.0, IIRC. But then, hw assist or no, x86 task gates "aren't that fast".

The SPARC context switch code, to me, always was very complex. The hardware had so much "sharing" (the register window set could split to multiple owners, so would the TSB/TLB, and the "MMU" was elaborate software in sparcv9 amyway). SPARC's achilles heel always were the "spills" - needless register window (and other cpu state) to/from memory. I'm kinda still curious from a "historical" point of view - thanks!