D150 - Guest threads and reported topology

Status: decided Date: 2026-08-20

The guest decides how many threads exist and each is a host thread; the host decides how fast they run. CpuTopology reports the target's shape by default, not the host's. Affinity follows AffinityPolicy - Observe by default, Map folds guest cores modulo the host's, Strict applies or refuses - and the request is recorded whichever applies.

Why: a slower host runs the same program more slowly, which is correct rather than a failure. A title asking about cores is asking about the machine it was designed for. No title has been shown to depend on placement, so a mapping invented first would read as a measurement.

Rejected:

• Enforcing a minimum core count: refuses a correct, slower run.
• Reporting the host topology: sizes thread pools for an untested machine.
• Clamping out-of-range cores: merges threads the guest separated.