Re: [Qemu-devel] [RFC 0/3] VirtIO RDMA

[Yuval Shaia]

On Fri, Apr 19, 2019 at 01:16:06PM +0200, Hannes Reinecke wrote:
> On 4/15/19 12:35 PM, Yuval Shaia wrote:
> > On Thu, Apr 11, 2019 at 07:02:15PM +0200, Cornelia Huck wrote:
> > > On Thu, 11 Apr 2019 14:01:54 +0300
> > > Yuval Shaia <address@hidden> wrote:
> > > 
> > > > Data center backends use more and more RDMA or RoCE devices and more and
> > > > more software runs in virtualized environment.
> > > > There is a need for a standard to enable RDMA/RoCE on Virtual Machines.
> > > > 
> > > > Virtio is the optimal solution since is the de-facto para-virtualizaton
> > > > technology and also because the Virtio specification
> > > > allows Hardware Vendors to support Virtio protocol natively in order to
> > > > achieve bare metal performance.
> > > > 
> > > > This RFC is an effort to addresses challenges in defining the RDMA/RoCE
> > > > Virtio Specification and a look forward on possible implementation
> > > > techniques.
> > > > 
> > > > Open issues/Todo list:
> > > > List is huge, this is only start point of the project.
> > > > Anyway, here is one example of item in the list:
> > > > - Multi VirtQ: Every QP has two rings and every CQ has one. This means 
> > > > that
> > > >    in order to support for example 32K QPs we will need 64K VirtQ. Not 
> > > > sure
> > > >    that this is reasonable so one option is to have one for all and
> > > >    multiplex the traffic on it. This is not good approach as by design 
> > > > it
> > > >    introducing an optional starvation. Another approach would be multi
> > > >    queues and round-robin (for example) between them.
> > > > 
> Typically there will be a one-to-one mapping between QPs and CPUs (on the
> guest). So while one would need to be prepared to support quite some QPs,
> the expectation is that the actual number of QPs used will be rather low.
> In a similar vein, multiplexing QPs would be defeating the purpose, as the
> overall idea was to have _independent_ QPs to enhance parallelism.

Since Jason already addresses the issue then i'll skip it.

> 
> > > > Expectations from this posting:
> > > > In general, any comment is welcome, starting from hey, drop this as it 
> > > > is a
> > > > very bad idea, to yeah, go ahead, we really want it.
> > > > Idea here is that since it is not a minor effort i first want to know if
> > > > there is some sort interest in the community for such device.
> > > 
> > > My first reaction is: Sounds sensible, but it would be good to have a
> > > spec for this :)
> > > 
> > > You'll need a spec if you want this to go forward anyway, so at least a
> > > sketch would be good to answer questions such as how many virtqueues
> > > you use for which purpose, what is actually put on the virtqueues,
> > > whether there are negotiable features, and what the expectations for
> > > the device and the driver are. It also makes it easier to understand
> > > how this is supposed to work in practice.
> > > 
> > > If folks agree that this sounds useful, the next step would be to
> > > reserve an id for the device type.
> > 
> > Thanks for the tips, will sure do that, it is that first i wanted to make
> > sure there is a use case here.
> > 
> > Waiting for any feedback from the community.
> > 
> I really do like the ides; in fact, it saved me from coding a similar thing
> myself :-)

Isn't it the great thing with open source :-)

> 
> However, I'm still curious about the overall intent of this driver. Where
> would the I/O be routed _to_ ?
> It's nice that we have a virtualized driver, but this driver is
> intended to do I/O (even if it doesn't _do_ any I/O ATM :-)
> And this I/O needs to be send to (and possibly received from)
> something.

Idea is to have a virtio-rdma device emulation (patch #2) on host that will
relay the traffic to the real HW on host.

It will be good to have design that will allow Virtio-HW to be plugged to
the host and use the same driver. In this case the emulated device would
not be needed - the driver will "attach" to the Virtqueue exposed by the
virtio-HW instead of the emulated RDMA device.

I don't know of any public virtio-rdma HW.

> 
> So what exactly is this something?
> An existing piece of HW on the host?
> If so, wouldn't it be more efficient to use vfio, either by using SR-IOV or
> by using virtio-mdev?

vfio needs to be implemented by every HW vendor where this approach is a
generic one that is not depended on the HW.

SV-IOV has it's limitations.

And with virtio-mdev, sorry but do not know, can you elaborate more?

> 
> Another guest?

No

> If so, how would we route the I/O from one guest to the other?
> Shared memory? Implementing a full-blown RDMA switch in qemu?
> 
> Oh, and I would _love_ to have a discussion about this at KVM Forum.
> Maybe I'll manage to whip up guest-to-guest RDMA connection using ivshmem
> ... let's see.

Well, I've posted a proposal for a talk, lets see if it'll be accepted.

> 
> Cheers,
> 
> Hannes
> -- 
> Dr. Hannes Reinecke            Teamlead Storage & Networking
> address@hidden                              +49 911 74053 688
> SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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