Re: Outline for VHOST_USER_PROTOCOL_F_VDPA

[Stefan Hajnoczi]

On Mon, Sep 28, 2020 at 03:21:56PM +0400, Marc-André Lureau wrote:
> On Mon, Sep 28, 2020 at 1:25 PM Stefan Hajnoczi <stefanha@redhat.com wrote:
> > Where this converges with multi-process QEMU
> > --------------------------------------------
> > At this point QEMU can run ad-hoc vhost-user backends using existing
> > VIRTIO device models. It is possible to go further by creating a
> > qemu-dev launcher executable that implements the vhost-user spec's
> > "Backend program conventions". This way a minimal device emulator
> > executable hosts the device instead of a full system emulator.
> >
> > The requirements for this are similar to the multi-process QEMU effort,
> > which aims to run QEMU devices as separate processes. One of the main
> > open questions is how to design build system and Kconfig support for
> > building minimal device emulator executables.
> >
> > In the case of vhost-user-net the qemu-dev-vhost-user-net executable
> > would contain virtio-net-device, vhost-user-backend, any netdevs the
> > user wishes to include, a QMP monitor, and a vhost-user backend
> > command-line interface.
> >
> > Where does this leave us? QEMU's existing VIRTIO device models can be
> > used as vhost-user devices and run in a separate processes from the VMM.
> > It's a great way of reusing code and having the flexibility to deploy it
> > in the way that makes most sense for the intended use case.
> >
> 
> My understanding is that this would only be able to expose virtio
> devices from external processes. But vfio-user could expose more kinds
> of devices, including the virtio devices.
> 
> Shouldn't we focus on vfio-user now, as the general out-of-process
> device solution?

Eventually vfio-user can replace vhost-user. However, vfio-user
development will take longer so for anyone already comfortable with
vhost-user I think extending the protocol with vDPA ioctls is
attractive.

Maybe we can get more organized around vfio-user and make progress
quicker?

Stefan

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