Re: [Qemu-devel] [PATCH] virtio: set virtio-net/virtio-mmio host features

[Peter Maydell]

On 20 February 2014 19:09, Mario Smarduch <address@hidden> wrote:
> Questionable in this patch - is cutting through several layers to set
> proxy properties. They should be set from "device" instance init
> before it's realized. The problem though is that unlike PCI
> that sets proxy and virtio-net properties via its virtio_net_properites[],
> the virtio-mmio version instantiates the proxy in the machine model,
> so it doesn't appear to be good place to set virtio-net
> host features since you don't know what virtio device will be plugged
> in later.

I think this function is the right place to set these properties,
yes. What I'm saying is that I don't see why you're doing it
this way rather than using the existing per-backend hook.
Maybe there's a reason not to use that hook, but you don't say.

> It's virtio_net_properties[] can only set virtio-net
> properites when it's instantiated. I think the proper way would
> be to refactor virtio-mmio to similar structure PCI version uses then
> one virtio_net_properties[] can be used selecting PCI or virtio-mmio at
> compile time. But right now it's unclear to me how pci and virtio-mmio
> versions intend to co-exist.

This is the result of a refactoring last year to allow virtio-mmio.
Basically the underlying structure now is:

 [some virtio transport device] <- virtio bus -> [virtio backend]

where the transport could be mmio, PCI, CCW, etc, and the
backend is net, blk, balloon, etc. We also provide devices
which are "virtio PCI transport plus a specific backend"
for (a) user convenience and (b) backwards compatibility, since
the pre-refactoring structure put the transport and backend
all in one lump. The all-in-one-lump arrangement is legacy:
we shouldn't break it, but it's not the model for virtio-mmio
to follow either.

The transport shouldn't know anything specific about the
backend it's plugged into (or vice-versa), but it's fine
for there to be hook functions so the transport and backend
can call each other at the appropriate times.

thanks
-- PMM