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

[Yuval Shaia]

On Fri, Apr 12, 2019 at 03:21:56PM +0530, Devesh Sharma wrote:
> On Thu, Apr 11, 2019 at 11:11 PM Yuval Shaia <address@hidden> wrote:
> >
> > On Thu, Apr 11, 2019 at 08:34:20PM +0300, Yuval Shaia wrote:
> > > On Thu, Apr 11, 2019 at 05:24:08PM +0000, Jason Gunthorpe 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.
> > > > > >
> > > > > > 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 :)
> > > >
> > > > I'm unclear why you'd want to have a virtio queue for anything other
> > > > that some kind of command channel.
> > > >
> > > > I'm not sure a QP or CQ benefits from this??
> > >
> > > Virtqueue is a standard mechanism to pass data from guest to host. By
> >
> > And vice versa (CQ?)
> >
> > > saying that - it really sounds like QP send and recv rings. So my thought
> > > is to use a standard way for rings. As i've learned this is how it is used
> > > by other virtio devices ex virtio-net.
> > >
> > > >
> > > > Jason
> > >
> I would like to ask more basic question, how virtio queue will glue
> with actual h/w qps? I may be to naive though.

Have to admit - I have no idea.
This work is based on emulated device so i'm my case - the emulated device
is creating the virtqueue. I guess that HW device will create a QP and
expose a virtqueue interface to it.
The same driver should serve both the SW and HW devices.

One of the objectives of this RFC is to collaborate an effort and
implementation notes/ideas from HW vendors.

> 
> -Regards
> Devesh