Apparatus and method for ensuring forward progress in coherent I/O systems

摘要

A snapshot mechanism that includes an apparatus and method for tracking DMA read requests for cacheable data that can be altered before the data is returned to a requesting I/O device is herein disclosed. Attributes that uniquely identify the original I/O device and DMA read request are stored in a cache tag unit. A read lock is set when a request is made to obtain the requested data when it is not resident in a local cache. When the cache line containing the requested data is snooped out and the read lock is set, then the cache line is set in a snapshot state. The snapshot state assures that only the original I/O device receives the read data when it has been altered subsequent to the time the original DMA read request was made. Once the data is returned to the original I/O device, the cache line is invalidated in order to prevent another I/O device from reading the stale data. Prefetched data is marked as such and cannot be marked as snapshot data.