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    SubjectRe: [Pv-drivers] [PATCH] SCSI driver for VMware's virtual HBA - V4.
    On Thu, 2009-09-10 at 16:43 -0700, Alok Kataria wrote:
    > Hi Anthony,
    > On Wed, 2009-09-09 at 15:12 -0700, Anthony Liguori wrote:
    > > Alok Kataria wrote:
    > > > I see your point, but the ring logic or the ABI that we use to
    > > > communicate between the hypervisor and guest is not shared between our
    > > > storage and network drivers. As a result, I don't see any benefit of
    > > > separating out this ring handling mechanism, on the contrary it might
    > > > just add some overhead of translating between various layers for our
    > > > SCSI driver.
    > > >
    > >
    > > But if you separate out the ring logic, it allows the scsi logic to be
    > > shared by other paravirtual device drivers. This is significant and
    > > important from a Linux point of view.
    > >
    > > There is almost nothing vmware specific about the vast majority of your
    > > code. If you split out the scsi logic, then you will receive help
    > > debugging, adding future features, and improving performance from other
    > > folks interested in this. In the long term, it will make your life
    > > much, much easier by making the driver relevant to a wider audience :-)
    > >From what you are saying, it seems that for that matter any physical
    > SCSI HBA's driver could be converted to use the virtio interface;
    > doesn't each and every driver have something like a ring/queue & I/O
    > register mechanism to talk to the device ?
    > Also, why would you add overhead for translation layers between APIs or
    > data structures just for the sake of it ? I guess you would say it helps
    > code re-usability, but I fail to see how much of a benefit that is. The
    > vast majority of the 1500 odd lines of the driver are still very
    > specific and tied to our PCI device and register interface.
    > I will just like to re-iterate this once again, this driver should be
    > treated no different than a hardware SCSI HBA driver, albeit a very
    > simple HBA. We export a PCI device as any other physical HBA, also the
    > driver talks to the device (emulation) through device IO regisers
    > without any hypercalls.
    > As for the virt_scsi layer, we can evaluate it whenever it is ready for
    > upstream and then take a more informed decision whether we should switch
    > to using it.
    > >
    > > > Having said that, I will like to add that yes if in some future
    > > > iteration of our paravirtualized drivers, if we decide to share this
    > > > ring mechanism for our various device drivers this might be an
    > > > interesting proposition.
    > > >
    > >
    > > I am certainly not the block subsystem maintainer, but I'd hate to see
    > > this merged without any attempt at making the code more reusable. If
    > > adding the virtio layering is going to somehow hurt performance, break
    > > your ABI, or in some way limit you, then that's something to certainly
    > > discuss and would be valid concerns. That said, I don't think it's a
    > > huge change to your current patch and I don't see any obvious problems
    > > it would cause.
    > >
    > I will also like to add that, this is just a driver and is isolated from
    > the rest of the core of the kernel. The driver is not doing anything
    > improper either and is using the SCSI stack the way that any other SCSI
    > driver would use it.
    > In earlier cases, when there were changes to the core kernel parts (e.g.
    > VMI, hypervisor_init- the tsc freq part) VMware did work with the
    > community to come up with generic interfaces.
    > In this case though, I don't think the advantages of using the virtio
    > interfaces are justified as yet. As and when the virt-scsi layer is
    > implemented we can re-evaluate our design and use that layer instead.
    > Holding inclusion of pvscsi driver until the development of virt-scsi
    > interface is completed doesn't sound right to me.

    Hi James,

    Please let us know your views on how should we proceed on this ? In my
    previous mail, I have tried to state my reservations against going the
    virt-scsi way just as yet. Also please note that we emulate a pure PCI
    device hence I don't see a reason why our driver should be treated any
    different than a driver for a physical HBA.


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