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

[Yuval Shaia]

On Thu, Apr 11, 2019 at 05:40:26PM +0000, Jason Gunthorpe 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
> > 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.
> 
> I doubt you can use virtio queues from userspace securely? Usually
> needs a dedicated page for each user space process

Not yet started to do any work on datapath, i guess you are right but need
further work on this area.
Thanks for raising the concern.

As i said, there are many open issues at this stage.

> 
> Jason