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

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Message-Id: <1251823925.3864.216.camel@mulgrave.site>
Date:	Tue, 01 Sep 2009 11:52:05 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Dmitry Torokhov <dtor@...are.com>
Cc:	Matthew Wilcox <matthew@....cx>, Roland Dreier <rdreier@...co.com>,
	Bart Van Assche <bvanassche@....org>,
	Alok Kataria <akataria@...are.com>,
	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 09:33 -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.

James


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