[Stgt-devel] Question for pass-through target design

[Robert Jennings]

* FUJITA Tomonori (fujita.tomonori at lab.ntt.co.jp) wrote:
> From: Robert Jennings <rcj at linux.vnet.ibm.com>
> Subject: [Stgt-devel] Question for pass-through target design
> Date: Fri, 4 May 2007 11:07:12 -0500
> 
> > It looks like the pass-through target support is currently broken, at
> > least as I've checked for ibmvstgt, but I think it's a general problem.
> > I wanted to check my assumptions and get ideas.
> 
> Yeah, unfortunately, it works only with the iSCSI target driver (which
> runs in user space).
> 
> 
> > The code isn't allocating any memory to pass along to the sg code to store
> > the result of a read or data for a write.  Currently, dxferp for sg_io_hdr
> > or dout_xferp/din_xferp for sg_io_v4 are assigned to the value of uaddr,
> > which is set to 0 in kern_queue_cmd.  With the pointer set to NULL,
> > the pass-through target isn't going to function.  Even if we had memory
> > allocated, there isn't a means of getting data to be written via sg down
> > this code path.
> > 
> > What ideas are there as to how the data will get to user-space so that
> > we can use sg?
> 
> For kernel-space drivers, we don't need to go to user-space. We can do
> the pass-through in kernel space. I talked with James about this last
> year and he said that if the code is implemented cleanly, he would
> merges it into mainline.

That sounds fine, how can we make the lld in kernel space aware that the
target is pass-through?  

Should the kernel driver operate as usual and queue the command to tgtd
and then tgtd would see that it's a pass-through target and communicate
back to the lld that the command should be processed in-kernel?  I think
that's probably the simplest method of achieving this without making
the kernel drivers aware of all target configuration and status but it
would mean a trip to tgtd for every command.  Otherwise we'd want to
communicate every change in configuration for the targets to the kernel
lld's and any other operational changes for tgtd.  Thoughts?