Re: Outline for VHOST_USER_PROTOCOL_F_VDPA

[Jason Wang]

On 2020/9/28 下午11:32, Stefan Hajnoczi wrote:
> 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?


Similar question could be asked for vDPA(kernel) vs VFIO(kernel).


> 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.


My understanding is for vhost-user may advantages:

- well defined interface, this helps a lot for e.g live migration (cross 
migration among different vendors), backend disconnection, device 
failover and there will be no vendor lock
- high level abstraction, not tie to a specific bus implementation, 
micro VM that want to get rid of PCI can use MMIO transport

So it doesn't conflict with vfio(-user) which is more suitable for any 
vendor specific device (API)s.

Thanks


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