D403 - The system-version value a guest reads is a per-machine setting

Status: decided Date: 2026-08-30

The system-version value a guest reads through the kernel's direct version call is a per-machine setting, packed the way a guest compares it; zero means unset, and an unset value refuses the call rather than answering the lowest possible version.

Why: A guest tests this value against version-band boundaries and branches on it, so answering an untested default would silently select whichever code path serves the oldest system there is, in a way that reads as ordinary success. Refusing an unset value applies the same rule already used for the kernel's release string (D397), for the same reason.

Rejected:

• Defaulting the setting to zero and answering it: zero sits inside the lowest tested band, so it would not fail visibly, it would silently pick the oldest system's branch.