On 5/30/19 4:28 AM, Paolo Bonzini wrote:
This allows a list of requests to be issued, with the LLD only writing the hardware doorbell when necessary, after the last request was prepared. This is more efficient if we have lists of requests to issue, particularly on virtualized hardware, where writing the doorbell is more expensive than on real hardware. The use case for this is plugged IO, where blk-mq flushes a batch of requests all at once. The API is the same as for blk-mq, just with blk-mq concepts tweaked to fit the SCSI subsystem API: the "last" flag in blk_mq_queue_data becomes a flag in scsi_cmnd, while the queue_num in the commit_rqs callback is extracted from the hctx and passed as a parameter. The only complication is that blk-mq uses different plugging heuristics depending on whether commit_rqs is present or not. So we have two different sets of blk_mq_ops and pick one depending on whether the scsi_host template uses commit_rqs or not.
Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>
