Deprecate fpu_kern_thread()

[jtl]

Summary

We believe all of the in-tree uses of kernel FPU now support using FPU_KERN_NOCTX or allocated contexts.

In cases where FPU support is used multiple times across discrete function calls (none of which rely on the FPU state from the previous one), it seems wasteful to potentially save/restore that state across these calls when there are context switches. Instead, for cases like these with short enough run times, we can rely on FPU_KERN_NOCTX. If the context must be carried across multiple calls or the run time is too long for a critical section, we can use an allocated context.

Test Plan

In a short test on amd64 with one particular workload, this seems to produce no obvious regression. We are going to run a more detailed analysis over the weekend with that same workload.

[emaste]

share/man/man9/fpu_kern.9

163–164

I wonder if we should provide a reference to removed interfaces, for someone who comes across older examples or code? Might not be an issue as we may expect them to easily find (via search engine) this review or the associated commit.