Re: [Qemu-devel] [PATCH 0/4] virtio: Move host features to backends

[Peter Maydell]

On 17 April 2015 at 14:43, Cornelia Huck <address@hidden> wrote:
> On Fri, 17 Apr 2015 20:13:42 +0800
> Shannon Zhao <address@hidden> wrote:
>
> [Some questions may be silly, but I'm not familiar with the virtio-mmio
> code]
>
>> The reason to do this is that the virtio-net-device can't expose host
>> features to guest while using virtio-mmio. So the performance is low.
>
> So how does virtio-mmio expose any host features?

The features are properties of the backend, not the transport.
So for devices where we didn't set these up as "properties
exist on the backend and the compatibility transport+backend
wrapper devices just forward those properties to the backend",
you can't set the properties. We got this right for some of
the backends (eg blk) but not all of them, I think.

NB: I haven't looked at the code in these patches yet, so this is
just the way I think it ought to work... it's not clear to me
yet why we need to change the s390 transports in this patchset.

-- PMM