Re: VHOST_USER_PROTOCOL_F_VDPA

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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Hi Stefan,

On 8/21/20 1:08 PM, Stefan Hajnoczi wrote:
> The first vDPA ioctls have been added to the vhost-user protocol and I
> wonder if it's time to fully change the vhost-user protocol's focus to
> providing a full VIRTIO device model like vDPA does.
> 
> Initially vhost-user was just used for vhost-net. As a result it didn't
> need the full VIRTIO device model including the configuration space and
> device status register.
> 
> Over the years device-specific messages were added to extend vhost-user
> to cover more of the VIRTIO device model. vhost-user-blk needed
> configuration space support, for example.
> 
> The problem for VMMs and device backend implementors is that the
> protocol is currently positioned halfway between the original vhost-net
> approach and the full VIRTIO device model. Even if a VMM implements
> VHOST_USER_GET_CONFIG, it can only expect it to work with
> vhost-user-blk, not vhost-user-net. Similarly, a vhost-user-net device
> backend cannot implement VHOST_USER_GET_CONFIG and expect all VMMs to
> allow it to participate in configuration space emulation because
> existing VMMs won't send that message.
> 
> The current approach where each device type uses a undocumented subset
> of vhost-user messages is really messy. VMM and device backend
> implementors have to look at existing implementations to know what is
> expected for a given device type. It would be nice to switch to the
> VIRTIO device model so that the VIRTIO specification can be used as the
> reference for how device types work.
> 
> Now that vDPA is here and extends the kernel vhost ioctls with a full
> VIRTIO device model, it might be a good time to revise the vhost-user
> protocol.
> 
> A vdpa-user protocol (or vhost-user 2.0) would replace the current mess
> with a full VIRTIO device model. Both VMMs and device backends would
> require changes to support this, but it would be a cleaner path forward
> for the vhost-user protocol.
> 
> One way of doing this would be a new VHOST_USER_PROTOCOL_F_VDPA feature
> bit that indicates all the currently existing Linux vDPA ioctl messages
> are available. Legacy vhost-user messages with overlapping functionality
> must not be used when this bit is set. Most importantly, device backends
> need to implement the full VIRTIO device model, regardless of device
> type (net, blk, scsi, etc).
> 
> The device type most affected by this change would be virtio-net. The
> other device types are already closer to the full VIRTIO device model.
> 
> I wanted to share this idea in case someone is currently trying to
> figure out how to add more VIRTIO device model functionality to
> vhost-user.
> 
> Stefan
> 

I understand the need and like the idea.

Maxime


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