DAX support in virtiofsd

Hello,

My team has been using DAX w/ virtiofsd for read-only devices. We are
interested in continuing this use case, but support has been removed
upstream for

* setup/remove mapping in vhost-user rust crate.
https://github.com/rust-vmm/vhost/commit/569ef077facafe21f16c0e65ff826b26d3c9a0b3
discussion: https://github.com/rust-vmm/vhost/issues/213

* as a consequence, setupmapping/removemapping in virtiofsd.
https://gitlab.com/virtio-fs/virtiofsd/-/commit/87f67e84cb032e6f4ce4a9f408b6305f33d2d041

The virtio spec already/still contains support for DAX.

The primary reasons for removing support for this feature in vhost and
virtiofsd appear to be that it is not in the vhost-user spec
(https://www.qemu.org/docs/master/interop/vhost-user.html) and there
are major edge cases not dealt with, so DAX can't be safely used if
the backing store can change in size.

Some of the major technical challenges to supporting DAX were
discussed in: https://lore.kernel.org/virtio-fs/[email protected]/
    ...
Specifically:
> 1. How to inject SIGBUS when the guest accesses a page that's beyond
> the end-of-file.
> 2. Implementing the vhost-user messages for mapping ranges of files to
> the vhost-user frontend.

I expect the steps to support DAX in virtiofsd are:

1. add the necessary vhost-user messages for mapping ranges to the
spec ... What is the RFC process for modifying the vhost standard?
What mailing list should I use?

2. re-implement vhost-user messages in the vhost-user rust crate

3. re-implement setup/remove mapping in virtiofsd.

4. implement new vhost-user messages in qemu?

5. find solution for safely handling page faults beyond EOF.

Can you offer comments, guidance, opinions? Is there resistance to
this work or is it that no one was interested enough to complete it?