Re: [PATCH 1/2] scsi_host: add support for request batching

[Bart Van Assche <bvanassche@acm.org>]

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@acm.org>