linux-kernel - Re: [RFC v2 16/21] tcm: Add PSCSI subsystem plugin

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Message-Id: <1289296562.19946.88.camel@haakon2.linux-iscsi.org>
Date:	Tue, 09 Nov 2010 01:56:02 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Christie <michaelc@...wisc.edu>,
	Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
	James Bottomley <James.Bottomley@...e.de>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC v2 16/21] tcm: Add PSCSI subsystem plugin

On Tue, 2010-11-09 at 11:16 +0200, Boaz Harrosh wrote:
> On 11/09/2010 07:13 AM, Nicholas A. Bellinger wrote:
> > <More follow up on Boaz comments from last week>
> > 
> 
> <snip>
> 
> >>> +static inline void pscsi_blk_init_request(
> >>> +	struct se_task *task,
> >>> +	struct pscsi_plugin_task *pt,
> >>> +	struct request *req,
> >>> +	int bidi_read)
> >>> +{
> >>> +	/*
> >>> +	 * Defined as "scsi command" in include/linux/blkdev.h.
> >>> +	 */
> >>> +	req->cmd_type = REQ_TYPE_BLOCK_PC;
> >>> +	/*
> >>> +	 * For the extra BIDI-COMMAND READ struct request we do not
> >>> +	 * need to setup the remaining structure members
> >>> +	 */
> >>> +	if (bidi_read)
> >>> +		return;
> >>> +	/*
> >>> +	 * Setup the done function pointer for struct request,
> >>> +	 * also set the end_io_data pointer.to struct se_task.
> >>> +	 */
> >>> +	req->end_io = pscsi_req_done;
> >>> +	req->end_io_data = (void *)task;
> >>> +	/*
> >>> +	 * Load the referenced struct se_task's SCSI CDB into
> >>> +	 * include/linux/blkdev.h:struct request->cmd
> >>> +	 */
> >>> +	req->cmd_len = scsi_command_size(pt->pscsi_cdb);
> >>> +	req->cmd = &pt->pscsi_cdb[0];
> >>
> >> Here!   req->cmd = TASK_CMD(task)->t_task_cdb;
> >>
> >> I don't see in this patch the actual memcpy of TASK_CMD(task)->t_task_cdb into
> >> pt->pscsi_cdb, which I suspect is done in Generic code through the use of
> >> ->get_cdb(struct se_task *);?
> >>
> >> If so then it means all the plugins have their own copy of the CDB? Now I finally
> >> understand the get_max_cdb_len().
> >>
> >> All that could be totally clean. All plugins use the TASK_CMD(task)->t_task_cdb
> >> directly. Then get_max_cdb_len(), get_cdb(), the memcpy() and all these extra buffers
> >> just magically go away.
> > 
> > As described in the last response, the T_TASK(cmd)->t_task_cdb is used
> > as a base for all CDBs, and is the primary storage for all *non*
> > SCF_SCSI_DATA_SG_IO_CDB type ops.
> > 
> > For all bulk SCF_SCSI_DATA_SG_IO_CDB, we will memcpy
> > T_TASK(cmd)->t_task_cdb into the backend specific location (for
> > TCM/pSCSI this is pt->pscsi_cdb[])  and then update the LBA+length when
> > splitting the single received struct se_cmd across multiple struct
> > se_task w/ backend descriptors due to backend struct
> > se_dev_limits->limits.max_sectors limitations for underlying backend HW.
> > 
> 
> OK Thanks I was missing this part. Sorry I only looked at the patches and
> not the complete code.
> 
> So for pSCSI could you use the cmnd buffer at struct request? Maybe
> postpone a little bit the patching of the cdb to after the request allocation.

<nod>  Again, this would only be for the non SCF_SCSI_DATA_SG_IO_CDB
cases for TCM/pSCSI, so I am not sure if the complexity is worth the
benefit for the non SCF_SCSI_DATA_SG_IO_CDB descriptors..

> Than anything bigger then 16 bytes (max cmnd at struct request) you allocate
> and free on request completion.

Hmm yeah, that adds a bit of extra complexity for TCM/pSCSI considering
the default TCM_MAX_COMMAND_SIZE=32

> But all that could be cleaned up later. You are currently busy with bigger
> stuff, just as a future note.
> 

<nod> point taken.

> The rest look good. I have some holes in my knowledge of how you did bidi.
> I promise to one of these days actually clone the tree and look at the real
> code.
> My goal is to have Lio-iscsi => lio-pSCSI => iscsi-initiator as a passthrough
> of OSD commands. That will prove things are done right.
> 

Excellent, please let me know if you have any questions getting setup.

> Also a Lio-iscsi => lio-tgt_if => tgtd-user-mode should also be very useful
> and OSD-able
> 

Indeed, I am still planning to spend a day or two before the end of the
year to get the basic pieces running for target_core_stgt.c and the main
I/O path for a userspace passthrough backend module function.

As usual, I am happy to test and accept patches before then..  ;)

> Thanks, Nic. Hope you feeling better now
> Boaz

Most certainly.  Thanks again for your detailed review!

--nab


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