Re: [Qemu-devel] [PULL 5/5] hw/arm/virt: Support legacy -nic command line syntax

[Alex Williamson]

On Mon, 2016-01-18 at 15:54 +0100, Eric Auger wrote:
> On 01/18/2016 03:14 PM, Peter Maydell wrote:
> > On 18 January 2016 at 13:57, Eric Auger <address@hidden>
> > wrote:
> > > Hi Peter,
> > > On 01/18/2016 02:34 PM, Peter Maydell wrote:
> > > > Hmm, I guess this is changing things in that we now will have a
> > > > virtio PCI device appearing if you use the default (-net nic
> > > > -net user)
> > > > settings. But I don't see why that would particularly interfere
> > > > with VFIO passthrough, except in as much as the guest now has
> > > > two network cards in it and might be preferring one as eth0
> > > > rather than the other...
> > > Yes that's what currently happens I think. I get the slirp thing
> > > on eth0
> > > and my passthrough'ed device on eth1. That's not very
> > > straightforward
> > > for the end-user to get those 2 NIC's now. In case I passthrough
> > > some
> > > NIC's I would have expected this default NIC not be instantiated?
> > 
> > The QEMU networking layer only knows about networking controlled
> > by the -net or -netdev options (and in those cases it does disable
> > the default network device). Because the back-end for passthrough
> > NICs is completely unknown to QEMU (it is all done in hardware),
> > I'm not sure we have any way to know that the thing you've passed
> > through
> > is a NIC and not some other random PCI device...
> > 
> > > Alex, how do you manage on x86 platforms with VFIO-PCI NIC?
> > 
> > ...but presumably the x86 folks have been here before us
> > and know how this should work :-)

No, we haven't.  vfio-pci devices are opaque on x86, we don't know or
care that they're NICs or HBAs or GPUs or whatever.  If you don't want the 
default NIC, turn it off with -net none.  Don't want graphics, -nographics.  
Default QEMU devices may be useful for the commandline, but do they really 
matter in practical use?

> Yes. maybe we could create a new network backend VFIO and use that kind
> of cmd line:
> 
> NET_OPTIONS="-netdev VFIO,id=myeth0 \
> -device vfio-amd-xgbe,host=e0900000.xgmac,netdev=myeth0"
> 
> In the specialized VFIO-Platform device I can easily add a dummy NICConf
> field and add
> 
> static Property amd_xgbe_properties[] = {
>     DEFINE_NIC_PROPERTIES(VFIOAmdXgbeDevice, conf),
>     DEFINE_PROP_END_OF_LIST(),
> };
> 
> But it complexifies the user command line quite a lot and not sure it is
> worth the candle?

In the general case, I don't see vfio-pci going in this direction.  We
do not want to know this much about the device.  Thanks,

Alex