Re: [Qemu-devel] Hitting 29 NIC limit

[Markus Armbruster <armbru@redhat.com>]

Avi Kivity <avi@redhat.com> writes:

>  On 10/14/2010 12:54 AM, Anthony Liguori wrote:
>> On 10/13/2010 05:32 PM, Anjali Kulkarni wrote:
>>>
>>> Hi,
>>>
>>> Using the legacy way of starting up NICs, I am hitting a limitation
>>> after 29
>>> NICs ie no more than 29 are detected (that's because of the 32 PCI slot
>>> limit on a single bus- 3 are already taken up)
>>> I had initially increased the MAX_NICS to 48, just on my tree, to get to
>>> more, but ofcource that wont work.
>>> Is there any way to go beyond 29 NICs the legacy way?  What is the
>>> maximum
>>> that can be supported by the qdev mothod?
>>
>> I got up to 104 without trying very hard using the following script:
>>
>> args=""
>> for slot in 5 6 7 8 9 10 11 12 13 14 15 16 17; do
>> for fn in 0 1 2 3 4 5 6 7; do
>>     args="$args -netdev user,id=eth${slot}_${fn}"
>>     args="$args -device
>> virtio-net-pci,addr=${slot}.${fn},netdev=eth${slot}_${fn},multifunction=on,romfile="
>> done
>> done
>>
>> x86_64-softmmu/qemu-system-x86_64 -hda ~/images/linux.img ${args}
>> -enable-kvm
>>
>> The key is to make the virtio-net devices multifunction and to fill
>> out all 8 functions for each slot.

I'm amazed that works.  Can't see how creating another qdev in the same
slot makes a proper multifunction device.

> This is unlikely to work right wrt pci hotplug.  If we want to support
> a large number of interfaces, we need true multiport cards.

Indeed.  As far as I know, we can't hot plug multifunction PCI devices.

> What's the motivation for such a huge number of interfaces?