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

From: James Bottomley
Date: Wed Sep 02 2009 - 11:06:26 EST

On Tue, 2009-09-01 at 19:55 -0700, Alok Kataria wrote:
> On Tue, 2009-09-01 at 11:15 -0700, James Bottomley wrote:
> > On Tue, 2009-09-01 at 10:41 -0700, Alok Kataria wrote:
> > > > lguest uses the sg_ring abstraction. Xen and KVM were certainly looking
> > > > at this too.
> > >
> > > I don't see the sg_ring abstraction that you are talking about. Can you
> > > please give me some pointers.
> >
> > it's in drivers/lguest ... apparently it's vring now and the code is in
> > driver/virtio
> >
> > > Also regarding Xen and KVM I think they are using the xenbus/vbus
> > > interface, which is quite different than what we do here.
> >
> > Not sure about Xen ... KVM uses virtio above.
> >
> > > >
> > > > > 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.
> I don't see how, the rest of the code comprises of IO/MMIO space & ring
> processing which is very different in each of the implementations. What
> is left is the setup and initialization code which obviously depends on
> the implementation of the driver data structures.

Are there benchmarks comparing the two approaches?

> > > And not just that, different HV-vendors can have different features,
> > > like say XYZ can come up tomorrow and implement the multiple rings
> > > interface so the feature set doesn't remain common and we will have less
> > > code to share in the not so distant future.
> >
> > Multiple rings is really just a multiqueue abstraction. That's fine,
> > but it needs a standard multiqueue control plane.
> >
> > The desire to one up the competition by adding a new whiz bang feature
> > to which you code a special interface is very common in the storage
> > industry. The counter pressure is that consumers really like these
> > things standardised. That's what the transport class abstraction is all
> > about.
> >
> > We also seem to be off on a tangent about hypervisor interfaces. I'm
> > actually more interested in the utility of an SRP abstraction or at
> > least something SAM based. It seems that in your driver you don't quite
> > do the task management functions as SAM requests, but do them over your
> > own protocol abstractions.
> Okay, I think I need to take a step back here and understand what
> actually are you asking for.
> 1. What do you mean by the "transport class abstraction" ?
> Do you mean that the way we communicate with the hypervisor needs to be
> standardized ?

Not really. Transport classes are designed to share code and provide a
uniform control plane when the underlying implementation is different.

> 2. Are you saying that we should use the virtio ring mechanism to handle
> our request and completion rings ?

That's an interesting question. Virtio is currently the standard linux
guest<=>hypervisor communication mechanism, but if you have comparative
benchmarks showing that virtual hardware emulation is faster, it doesn't
need to remain so.

> We can not do that. Our backend expects that each slot on the ring is
> in a particular format. Where as vring expects that each slot on the
> vring is in the vring_desc format.

Your backend is a software server, surely?

> 3. Also, the way we communicate with the hypervisor backend is that the
> driver writes to our device IO registers in a particular format. The
> format that we follow is to first write the command on the
> COMMAND_REGISTER and then write a stream of data words in the
> DATA_REGISTER, which is a normal device interface.
> The reason I make this point is to highlight we are not making any
> hypercalls instead we communicate with the hypervisor by writing to
> IO/Memory mapped regions. So from that perspective the driver has no
> knowledge that its is talking to a software backend (aka device
> emulation) instead it is very similar to how a driver talks to a silicon
> device. The backend expects things in a certain way and we cannot
> really change that interface ( i.e. the ABI shared between Device driver
> and Device Emulation).
> So sharing code with vring or virtio is not something that works well
> with our backend. The VMware PVSCSI driver is simply a virtual HBA and
> shouldn't be looked at any differently.
> Is their anything else that you are asking us to standardize ?

I'm not really asking you to standardise anything (yet). I was more
probing for why you hadn't included any of the SCSI control plane
interfaces and what lead you do produce a different design from the
current patterns in virtual I/O. I think what I'm hearing is "Because
we didn't look at how modern SCSI drivers are constructed" and "Because
we didn't look at how virtual I/O is currently done in Linux". That's
OK (it's depressingly familiar in drivers), but now we get to figure out
what, if anything, makes sense from a SCSI control plane to a hypervisor
interface and whether this approach to hypervisor interfaces is better
or worse than virtio.


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