Draining operation for draining dirty cache lines to persistent memory

An apparatus comprising:

• means for processing instructions, supporting processing of a transaction comprising instructions processed speculatively between a transaction start instruction and a transaction end instruction, for which the means for processing is configured to prevent commitment of results of the speculatively processed instructions until the transaction end instruction is reached in the absence of the transaction being aborted;
• and means for caching data in a cache hierarchy comprising at least two levels of cache;
• in which:
• in response to a store instruction processed within a transaction, the means for processing is configured to write speculative store data to the means for caching with the speculative store data marked as speculative to prevent the speculative store data passing beyond a predetermined level cache in the cache hierarchy to at least one further level cache until the transaction is committed;
• and
• the apparatus comprises means for draining cached data to persistent memory, where in response to detecting a draining trigger event having potential to cause loss of state stored in the at least one further level cache, the means for draining is configured to perform a draining operation to scan a subset of the cache hierarchy to identify dirty cache lines and write data associated with the dirty cache lines to the persistent memory, the subset of the cache hierarchy comprising the at least one further level cache, where in the draining operation, the means for draining is configured to ensure that dirty cache lines storing speculative store data marked as speculative in the predetermined level cache are prevented from being drained to the persistent memory.

A method comprising:

• processing a transaction comprising instructions processed speculatively between a transaction start instruction and a transaction end instruction, for which commitment of results of the speculatively processed instructions is prevented until the transaction end instruction is reached in the absence of the transaction being aborted;
• in response to a store instruction processed within a transaction, writing speculative store data to a cache hierarchy comprising at least two levels of cache, with the speculative store data marked as speculative to prevent the speculative store data passing beyond a predetermined level cache in the cache hierarchy to at least one further level cache until the transaction is committed;
• and
• in response to detection of a draining trigger event having potential to cause loss of state stored in the at least one further level cache, performing a draining operation to scan a subset of the cache hierarchy to identify dirty cache lines and write data associated with the dirty cache lines to persistent memory, the subset of the cache hierarchy comprising the at least one further level cache, where when performing the draining operation, dirty cache lines storing speculative store data marked as speculative in the predetermined level cache are prevented from being drained to the persistent memory.