Intel® Pentium® Silver and Intel® Celeron® Processors Datasheet, Volume 1
SMBus Power Gating
SMBus shares the Power Gating Domain with Primary-to-Sideband Bridge (P2SB).
A single FET controls the single Power Gating Domain; but SMBus and P2SB each has its own dedicated Power Gating Control Block.
The FET is only turned off when all these interfaces are ready to PG entry or already in the PG state.