cpu_auxmsr: assert caller is preventing CPU migration

[adam_fenn.io]

Test Plan

Peformed the following sanity-checks:

• i386
  • Confirmed assertion is not triggered during boot of a 4-CPU system (a bhyve VM).
• amd64
  • Confirmed assertion is not triggered during boot of a 4-CPU system (a VMWare VM).
  • Confirmed assertion is not triggered in the VM entry/exit code path (used an invocation of dtrace -f fbt:kernel:cpu_auxmsr to confirm this routine was, as expected, being called while a bhyve VM was running).

The resume codepaths were not sanity-checked, but, by inspection, these appear equally clear as contexts with interrupts disabled as the cases that were sanity-checked.

[kib]

sys/amd64/amd64/initcpu.c

224 ↗

(On Diff #76121)

I suggest to trim this sentence to just

Caller should prevent CPU migration.

235 ↗

(On Diff #76121)

I am not sure the comment is useful. If somebody is using this in context that only prevents scheduling, e.g. in critical section, he should be qualified enough to change assert to accommodate new situation. Comment above the function gives enough clue, if needed.

237 ↗

(On Diff #76121)

Indent of contig lines should be +4 spaces, no alignment to opening braces. Also I believe the message could be made much shorter without loosing the information.

[adam_fenn.io]

Incorporate @kib's review feedback.

[kib]

sys/amd64/amd64/initcpu.c

229 ↗

(On Diff #76127)

I suggest to write the message Context switch possible instead of interrupts enabled which just repeats the assert code. It is more to the point IMO.

230 ↗

(On Diff #76127)

Remove blank line.

[adam_fenn.io]

Incorporate @kib's review feedback.

[grehan]

Thanks kib@, I can commit this.