Subject: Re: [virtio-dev] Re: [Qemu-devel] [PATCH] qemu: Introduce VIRTIO_NET_F_STANDBY feature bit to virtio_net

From: Cornelia Huck <[email-masked]>
Date: Wed, 20 Jun 2018 11:53:59 +0200

On Tue, 19 Jun 2018 23:32:06 +0300
"Michael S. Tsirkin" <[email-masked]> wrote:

> On Tue, Jun 19, 2018 at 12:54:53PM +0200, Cornelia Huck wrote:
> > Sorry about dragging mainframes into this, but this will only work for
> > homogenous device coupling, not for heterogenous. Consider my vfio-pci
> > + virtio-net-ccw example again: The guest cannot find out that the two
> > belong together by checking some group ID, it has to either use the MAC
> > or some needs-to-be-architectured property.
> > 
> > Alternatively, we could propose that mechanism as pci-only, which means
> > we can rely on mechanisms that won't necessarily work on non-pci
> > transports. (FWIW, I don't see a use case for using vfio-ccw to pass
> > through a network card anytime in the near future, due to the nature of
> > network cards currently in use on s390.)  
> 
> That's what it boils down to, yes.  If there's need to have this for
> non-pci devices, then we should put it in config space.
> Cornelia, what do you think?
> 

I think the only really useful config on s390 is the vfio-pci network
card coupled with a virtio-net-ccw device: Using an s390 network card
via vfio-ccw is out due to the nature of the s390 network cards, and
virtio-ccw is the default transport (virtio-pci is not supported on any
enterprise distro AFAIK).

For this, having a uuid in the config space could work (vfio-pci
devices have a config space by virtue of being pci devices, and
virtio-net-ccw devices have a config space by virtue of being virtio
devices -- ccw devices usually don't have that concept).