Re: [Qemu-devel] [PATCH 3/4] qemu:virtio-net: Add support for qemu_vlan_rxfilter

[Paul Brook <paul@codesourcery.com>]

> - A device requests a filter and is told if the request is successful
>   - On success the device may skip it's own filtering
> - If another vlan client is added, the following _must_ occur:
>   - The "filterer" must clear (remove) the filter
>   - The "filteree" must revert to using their own filtering
> - If a vlan client is removed, the following _may_ occur:
>   - vlan clients are notified that they may retry their filter
>..
> The added()/removed() interface feels like the right solution to this.

I think your analysis is fundamentally flawed. Firstly I'm not sure the above 
holds when going between 1 and 2 clients on a vlan.  Even ignoring that, you 
are making implicit assumptions about when a filter will succeed. If these 
assumptions are broken (which seems likely if we ever implement filtering 
with more than 2 devices on a vlan) they you'll get subtle breakage in every 
single user of the API.

> We could use a changed() function, but it would need to know the
> direction of the change, which leads back to the same mechanics.  If
> there's a better way, please suggest it.  Thanks,

I still don't see why the device needs to know what's changed. The response 
should always be the same: Request a filter, and see if it works.

Paul