Re: [virtio-dev] [RFC] Upstreaming virtio-wayland (or an alternative)

[David Stevens <stevensd@chromium.org>]

On Thu, Feb 27, 2020 at 6:09 PM Boris Brezillon
<boris.brezillon@collabora.com> wrote:
>
> On Thu, 27 Feb 2020 13:20:51 +0900
> David Stevens <stevensd@chromium.org> wrote:
>
> > > * manage a central UUID <-> 'struct file' map that allows virtio-pipe
> > >   to convert FDs to UUIDs, pass UUIDs through a pipe and convert those
> > >   UUIDs back to FDs on the other end
> > >   - we need to expose an API to let each subsystem register/unregister
> > >     their UUID <-> FD mapping (subsystems are responsible for the UUID
> > >     creation/negotiation)
> >
> > Can you provide more detail about the envisioned scope of this
> > framework?
>
> The scope is "generic message+FD passing" interface, which is pretty
> much what virtio-wl provides.

I think that scope is too broad. A socket is a 'generic message+FD'
interface. Unless there's the expectation that the interface should
eventually be as flexible as a regular domain socket, I think it would
be a good idea to frame the scope of the interface more precisely.

Part of this ambiguity comes from the informal usage of the term 'FD'.
An FD is a concept in Linux and other operating systems (and not even
all operating systems - e.g. Fuchsia). At present, FDs are not a
concept in virtio. Talking about sending FDs over virtio handwaves a
lot of details about what that's actually going on.

> > How
> > do operations on the guest FD affect the host FD, and vice versa?
>
> Depends what you mean by operations. If we're talking about regular
> read/write/ioctl/mmap operations on the guest side, it's up to the
> subsystem/driver to implement the expected behavior.

I think part of my confusion comes from the fact that virtio-wayland
seems to provide both the IPC mechanism described for virtio-ipc as
well as some additional guest/host file sharing support. If that is
actually the case, then I guess a striped down version of
virtio-wayland would still be necessary.

-David