linux-kernel - Re: [PATCH] SCSI driver for VMware's virtual HBA.

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Message-Id: <1251824374.16169.128.camel@ank32.eng.vmware.com>
Date:	Tue, 01 Sep 2009 09:59:34 -0700
From:	Alok Kataria <akataria@...are.com>
To:	James Bottomley <James.Bottomley@...e.de>
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 09:52 -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. 

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.

Alok
> 
> James
> 
> 

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