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