Re: [Alacrityvm-devel] [GIT PULL] AlacrityVM guest drivers for2.6.33

[Ira W. Snyder]

On Thu, Dec 24, 2009 at 11:09:39AM -0600, Anthony Liguori wrote:
> On 12/23/2009 05:42 PM, Ira W. Snyder wrote:
> >
> > I've got a single PCI Host (master) with ~20 PCI slots. Physically, it
> > is a backplane in a cPCI chassis, but the form factor is irrelevant. It
> > is regular PCI from a software perspective.
> >
> > Into this backplane, I plug up to 20 PCI Agents (slaves). They are
> > powerpc computers, almost identical to the Freescale MPC8349EMDS board.
> > They're full-featured powerpc computers, with CPU, RAM, etc. They can
> > run standalone.
> >
> > I want to use the PCI backplane as a data transport. Specifically, I
> > want to transport ethernet over the backplane, so I can have the powerpc
> > boards mount their rootfs via NFS, etc. Everyone knows how to write
> > network daemons. It is a good and very well known way to transport data
> > between systems.
> >
> > On the PCI bus, the powerpc systems expose 3 PCI BAR's. The size is
> > configureable, as is the memory location at which they point. What I
> > cannot do is get notified when a read/write hits the BAR. There is a
> > feature on the board which allows me to generate interrupts in either
> > direction: agent->master (PCI INTX) and master->agent (via an MMIO
> > register). The PCI vendor ID and device ID are not configureable.
> >
> > One thing I cannot assume is that the PCI master system is capable of
> > performing DMA. In my system, it is a Pentium3 class x86 machine, which
> > has no DMA engine. However, the PowerPC systems do have DMA engines. In
> > virtio terms, it was suggested to make the powerpc systems the "virtio
> > hosts" (running the backends) and make the x86 (PCI master) the "virtio
> > guest" (running virtio-net, etc.).
> 
> IMHO, virtio and vbus are both the wrong model for what you're doing. 
> The key reason why is that virtio and vbus are generally designed around 
> the concept that there is shared cache coherent memory from which you 
> can use lock-less ring queues to implement efficient I/O.
> 
> In your architecture, you do not have cache coherent shared memory. 
> Instead, you have two systems connected via a PCI backplace with 
> non-coherent shared memory.
> 
> You probably need to use the shared memory as a bounce buffer and 
> implement a driver on top of that.
> 
> > I'm not sure what you're suggesting in the paragraph above. I want to
> > use virtio-net as the transport, I do not want to write my own
> > virtual-network driver. Can you please clarify?
> 
> virtio-net and vbus are going to be overly painful for you to use 
> because no one end can access arbitrary memory in the other end.
> 

The PCI Agents (powerpc's) can access the lowest 4GB of the PCI Master's
memory. Not all at the same time, but I have a 1GB movable window into
PCI address space. I hunch Kyle's setup is similar.

I've proved that virtio can work via my "crossed-wires" driver, hooking
two virtio-net's together. With a proper in-kernel backend, I think the
issues would be gone, and things would work great.

> > Hopefully that explains what I'm trying to do. I'd love someone to help
> > guide me in the right direction here. I want something to fill this need
> > in mainline.
> 
> If I were you, I would write a custom network driver.  virtio-net is 
> awfully small (just a few hundred lines).  I'd use that as a basis but I 
> would not tie into virtio or vbus.  The paradigms don't match.
> 

This is exactly what I did first. I proposed it for mainline, and David
Miller shot it down, saying: you're creating your own virtualization
scheme, use virtio instead. Arnd Bergmann is maintaining a driver
out-of-tree for some IBM cell boards which is very similar, IIRC.

In my driver, I used the PCI Agent's PCI BAR's to contain ring
descriptors. The PCI Agent actually handles all data transfer (via the
onboard DMA engine). It works great. I'll gladly post it if you'd like
to see it.

In my driver, I had to use 64K MTU to get acceptable performance. I'm
not entirely sure how to implement a driver that can handle
scatter/gather (fragmented skb's). It clearly isn't that easy to tune a
network driver for good performance. For reference, my "crossed-wires"
virtio drivers achieved excellent performance (10x better than my custom
driver) with 1500 byte MTU.

> > I've been contacted seperately by 10+ people also looking
> > for a similar solution. I hunch most of them end up doing what I did:
> > write a quick-and-dirty network driver. I've been working on this for a
> > year, just to give an idea.
> 
> The whole architecture of having multiple heterogenous systems on a 
> common high speed backplane is what IBM refers to as "hybrid computing". 
>   It's a model that I think will be come a lot more common in the 
> future.  I think there are typically two types of hybrid models 
> depending on whether the memory sharing is cache coherent or not.  If 
> you have coherent shared memory, the problem looks an awfully lot like 
> virtualization.  If you don't have coherent shared memory, then the 
> shared memory basically becomes a pool to bounce into and out-of.
> 

Let's say I could get David Miller to accept a driver as described
above. Would you really want 10+ seperate but extremely similar drivers
for similar boards? Such as mine, Arnd's, Kyle's, etc. It is definitely
a niche that Linux is lacking support for. And as you say, it is
growing.

It seems that no matter what I try, everyone says: no, go do this other
thing instead. Before I go and write the 5th iteration of this, I'll be
looking for a maintainer who says: this is the correct thing to be
doing, I'll help you push this towards mainline. It's been frustrating.

Ira
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