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

[Eric Auger]

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 :-)

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?

Thanks

Eric


> 
> thanks
> -- PMM
>