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SubjectRe: [Pv-drivers] [PATCH] SCSI driver for VMware's virtual HBA - V4.
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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.
Thanks,
Alok




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