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

FUJITA Tomonori fujita.tomonori
Fri May 4 19:04:30 CEST 2007

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.

Is anyone interested in implementing this?

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