[stgt] [PATCH] [tgt]: Convert bs_sg to use BSG v4 interface

[Alexander Nezhinsky]

> Alexander, I think that you added bs_sg.c. Dropping sg and supporting
> only bsg is ok for you? Or you still use this feature?

It is generally ok with me. 

I've used bs_sg mainly for some performance measurements.
As we had discussed once, it has a major drawback of not handling properly
cache_sync in some scenarios, because the pass-through mode is disabled. 
As far as i understand, the new implementation suffers from the same problem,
so it is subject to the same limitations and requires the same precautions.

The only other concern might be that if anybody uses bs_sg with real devices
then the bsg-based implementation may preclude using older kernels,
like 2.6.18 of RHEL5.x. 

As an alternative i could suggest having both bs_sg3 (old code) and 
bs_sg4 (new code), and compiling them in (both or sg3 only) conditioned on
a makefile flag like "make SG3=1".

A side question: is BSG documented anywhere?
I have read the kernel module's source, and it looks that it depends on
support for executing scsi commands in some layers below, 
within the block subsystem. Is the API for this is documented in any way?
I could not find any docs or discussions, only some patch submissions for BSG.
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