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

[Eric Auger <eric.auger@linaro.org>]

On 01/18/2016 04:55 PM, Alex Williamson wrote:
> 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 <eric.auger@linaro.org>
>>> 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. 

Ah OK. That's perfect then ;-)

Thanks

Eric
 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
>