Message-Id: <1251825949.12482.34.camel@mulgrave.site>
Date:	Tue, 01 Sep 2009 12:25:49 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	akataria@...are.com
Cc:	Dmitry Torokhov <dtor@...are.com>, Matthew Wilcox <matthew@....cx>,
	Roland Dreier <rdreier@...co.com>,
	Bart Van Assche <bvanassche@....org>,
	Robert Love <robert.w.love@...el.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Mike Christie <michaelc@...wisc.edu>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rolf Eike Beer <eike-kernel@...tec.de>,
	Maxime Austruy <maustruy@...are.com>
Subject: Re: [PATCH] SCSI driver for VMware's virtual HBA.

On Tue, 2009-09-01 at 09:59 -0700, Alok Kataria wrote:
> On Tue, 2009-09-01 at 09:52 -0700, James Bottomley wrote:
> > On Tue, 2009-09-01 at 09:33 -0700, Dmitry Torokhov wrote:
> > > On Tuesday 01 September 2009 09:16:51 am Matthew Wilcox wrote:
> > > > On Tue, Sep 01, 2009 at 09:12:43AM -0700, Roland Dreier wrote:
> > > > > I'm not really sure we should be trying to force drivers to share just
> > > > > because they are paravirtualized -- if there is real commonality, then
> > > > > sure put it in common code, but different hypervisors are probably as
> > > > > different as different hardware.
> > > > 
> > > > I really disagree.  This kind of virtualised drivers are pretty much
> > > > communication protocols, and not hardware.  As such, why design a new one?
> > > > If there's an infelicity in the ibmvscsi protocol, it makes sense to
> > > > design a new one.  But being different for the sake of being different
> > > > is just a way to generate a huge amount of make-work.
> > > > 
> > > 
> > > The same thing can be said about pretty much anything. We don't have
> > > single SCSI, network, etc driver handling every devices in their
> > > respective class, I don't see why it would be different here.
> > > A hypervisor presents the same interface to the guest OS (whether
> > > it is Linux, Solaris or another OS) much like a piece of silicone
> > > does and it may very well be different form other hypervisors.
> > 
> > Nobody said you had to have the exact same driver for every hypervisor.
> > What people are suggesting is that we look at commonalities in the
> > interfaces both from a control plane point of view (transport class) and
> > from a code sharing point of view (libscsivirt).  However, all the
> > hypervisor interfaces I've seen are basically DMA rings ... they really
> > do seem to be very similar across hypervisors, so it does seem there
> > could be a lot of shared commonality.  I'm not going to insist on RDMA
> > emulation, but perhaps you lot should agree on what a guest to
> > hypervisor DMA interface looks like.
> 
> Which is this other hypervisor driver that you are talking about,
> ibmvscsi is using RDMA emulation and I don't think you mean that. 

lguest uses the sg_ring abstraction.  Xen and KVM were certainly looking
at this too.

> And anyways how large is the DMA code that we are worrying about here ?
> Only about 300-400 LOC ? I don't think we might want to over-design for
> such small gains.

So even if you have different DMA code, the remaining thousand or so
lines would be in common.  That's a worthwhile improvement.

The benefit to users would be a common control plane and interface from
the transport class, plus common code means more testers regardless of
virtualisation technology chosen.

James


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