Re: [Qemu-devel] custom virt-io support (in user-mode-linux)

[Johannes Berg <johannes@sipsolutions.net>]

Hi Anton,

> > I'm thinking about adding virt-io support to UML, but the tricky part is
> > that while I want to use the virt-io basics (because it's a nice
> > interface from the 'inside'), I don't actually want the stock drivers
> > that are part of the kernel now (like virtio-net etc.) but rather
> > something that integrates with wifi (probably building on hwsim).

> I have looked at using virtio semantics in UML in the past around the 
> point when I wanted to make the recvmmsg/sendmmsg vector drivers common 
> in UML and QEMU. It is certainly possible,
> 
> I went for the native approach at the end though.

Hmm. I'm not sure what you mean by either :-)

Is there any commonality between the vector drivers? I can't see how
that'd work without a bus abstraction (like virtio) in qemu? I mean, the
kernel driver just calls uml_vector_sendmmsg(), which I'd say belongs
more to the 'outside world', but that can't really be done in qemu?

Ok, I guess then I see what you mean by 'native' though.

Similarly, of course, I can implement arbitrary virt-io devices - just
the kernel side doesn't call a function like uml_vector_sendmmsg()
directly, but instead the virt-io model, and the model calls the
function, which essentially is the same just with a (convenient)
abstraction layer.

But this leaves the fundamental fact the model code ("vector_user.c" or
a similar "virtio_user.c") is still part of the build.

I guess what I'm thinking is have something like "virtio_user_rpc.c"
that uses some appropriate RPC to interact with the real model. IOW,
rather than having all the model-specific logic actually be here (like
vector_user.c actually knows how to send network packets over a real
socket fd), try to call out to some RPC that contains the real model.

Now that I thought about it further, I guess my question boils down to
"did anyone ever think about doing RPC for Virt-IO instead of putting
the entire device model into the hypervisor/emulator/...".

johannes