Re: [PATCH 2/2] [tgt]: Add support for SG_IO CDB passthrough in scsi_cmd_perform()

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On Sun, 2010-05-30 at 12:02 +0300, Boaz Harrosh wrote:
> On 05/18/2010 08:13 AM, Nicholas A. Bellinger wrote:
> > From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
> > 
> > This patch changes usr/scsi.c:scsi_cmd_perform() to first check to see if
> > the struct backingstore_template->bs_passthrough=1 for SG_IO, then
> > check for a valid struct device_type_template->cmd_passthrough()
> > function pointer to determine if the device supports SG_IO.
> > 
> > This patch then updates the device_type in usr/sb.c to enable the functionality
> > for TYPE_DISK.
> > 
> > Signed-off-by: Nicholas A. Bellinger <nab@xxxxxxxxxxxxxxx>
> > ---
> >  usr/sbc.c  |    1 +
> >  usr/scsi.c |   11 +++++++++++
> >  usr/tgtd.h |    1 +
> >  3 files changed, 13 insertions(+), 0 deletions(-)
> > 
> > diff --git a/usr/sbc.c b/usr/sbc.c
> > index a048d53..d53f7f9 100644
> > --- a/usr/sbc.c
> > +++ b/usr/sbc.c
> > @@ -269,6 +269,7 @@ static struct device_type_template sbc_template = {
> >  	.lu_online	= spc_lu_online,
> >  	.lu_offline	= spc_lu_offline,
> >  	.lu_exit	= spc_lu_exit,
> > +	.cmd_passthrough = sbc_rw,
> >  	.ops		= {
> >  		{spc_test_unit,},
> >  		{spc_illegal_op,},
> > diff --git a/usr/scsi.c b/usr/scsi.c
> > index cef231b..1db786e 100644
> > --- a/usr/scsi.c
> > +++ b/usr/scsi.c
> > @@ -242,6 +242,17 @@ int scsi_cmd_perform(int host_no, struct scsi_cmd *cmd)
> >  
> >  	if (spc_access_check(cmd))
> >  		return SAM_STAT_RESERVATION_CONFLICT;
> > +	/* 
> > +	 * Use dev_type_template->cmd_passthrough() for usr/bs_sg.c devices
> > +	 */
> > +	if (cmd->dev->bst->bs_passthrough) {
> > +		if (!(cmd->dev->dev_type_template.cmd_passthrough)) {
> > +			eprintf("cmd->dev->dev_type_template.cmd_passthrough()"
> > +				" is NULL!\n");
> > +			return SAM_STAT_CHECK_CONDITION;
> > +		}
> > +		return cmd->dev->dev_type_template.cmd_passthrough(host_no, cmd);
> > +	}
> >  
> 
> Can't you just perform this check once somewhere and set cmd_perform to the
> new cmd_passthrough? why check for every command?
> 
> This is not accepted it is the hot path.

Good point..

> 
> >  	return cmd->dev->dev_type_template.ops[op].cmd_perform(host_no, cmd);
> >  }
> > diff --git a/usr/tgtd.h b/usr/tgtd.h
> > index 4f26a29..f8ee47f 100644
> > --- a/usr/tgtd.h
> > +++ b/usr/tgtd.h
> > @@ -108,6 +108,7 @@ struct device_type_template {
> >  	int (*lu_config)(struct scsi_lu *lu, char *args);
> >  	int (*lu_online)(struct scsi_lu *lu);
> >  	int (*lu_offline)(struct scsi_lu *lu);
> > +	int (*cmd_passthrough)(int, struct scsi_cmd *);
> >  
> 
> Actually you can do more:
> If you leave the checking and decisions to the target driver
> then you don't need to touch this file. Just let the device_type
> decide which vector to set once it knows what target is used.
> 
> (Like start with a vector that check and sets the vector to
>  the passthrough or the regular path, on first IO)
> 

Ok, this makes alot more sense for a STGT backstore passthrough.  I will
respin the patch to let device_type decide to setup which flavour of
->cmd_perform() it would like to use and drop the scsi_cmd_perform()
passthrough check, et al. for the fast path.

Thanks for your comments Boaz!

--nab



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