Subject: Re: [Scst-devel] Fwd: Re: linuxcon 2010...
From: James Bottomley <James.Bottomley@suse.de>
To: Vladislav Bolkhovitin <vst@vlnb.net>
Cc: "Nicholas A. Bellinger" <nab@linux-iscsi.org>,
        Dirk Meister <dmeister@uni-paderborn.de>, linux-scsi@vger.kernel.org,
        Chetan Loke <chetanloke@gmail.com>,
        Chetan Loke <generationgnu@yahoo.com>,
        scst-devel <scst-devel@lists.sourceforge.net>,
        linux-kernel@vger.kernel.org, Mike Christie <michaelc@cs.wisc.edu>
In-Reply-To: <4C73DA15.2010207@vlnb.net>
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Date: Tue, 24 Aug 2010 09:57:56 -0500
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On Tue, 2010-08-24 at 18:41 +0400, Vladislav Bolkhovitin wrote:
> James Bottomley, on 08/22/2010 12:43 AM wrote:
> > Interface re-use (or at least ABI compatibility) is the whole point,
> > it's what makes the solution a drop in replacement.
> 
> I see now. You want ABI compatibility to keep the "contract" that no 
> kernel changes can break applications binary compatibility for unlimited 
> time.
> 
> OK, we will write the compatibility module. It shouldn't take much time.
> 
> But before we start, I'd like to clear 2 related questions:
> 
> 1. How far the ABI compatibility "contract" goes? Are there cases, where 
> it isn't so strong? I'm asking, because I can recall that open-iscsi at 
> least once has broken ABI compatibility with user space tools. Was it an 
> accidental (but not corrected) mistake or was it deliberate? If the 
> latter, then, I guess, there must be some exceptions defining when ABI 
> compatibility can be not followed.

I don't think it has to be complete.  As long as the STGT people think
it's good enough, that's fine by me.

> 2. Currently STGT in the kernel is just 2 files, scsi_tgt_if.c and
> scsi_tgt_lib.c (with headers), + ibmvstgt driver. The C files define the 
> STGT interface in question. So, if we keep ABI compatibility with the 
> new target engine, we would have to keep those 2 files included in the 
> kernel,

This isn't really correct.  The ABI is defined by the headers not the
implementation.

>  which would effectively mean that STGT would stay in the kernel. 
> This would lead to the situation you are trying to avoid: 2 SCSI target 
> infrastructures in the kernel. Would it be OK?

If you mean is the marketing solution of wedging two products into the
kernel and calling it a single one going to fly, the answer is no.

James


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