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. James -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
