Re: [Qemu-devel] Hitting 29 NIC limit

[Anthony Liguori <anthony@codemonkey.ws>]

On 10/14/2010 07:10 AM, Daniel P. Berrange wrote:
> On Thu, Oct 14, 2010 at 02:07:17PM +0200, Avi Kivity wrote:
>    
>>   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.
>>>        
>> This is unlikely to work right wrt pci hotplug.  If we want to support a
>> large number of interfaces, we need true multiport cards.
>>      
> Or a PCI bridge to wire up more PCI buses, so we raise the max limit for
> any type of device we emulate.
>    

I've always been sceptical of this.  When physical systems have a large 
number of NICs, it's via multiple functions, not a bunch of PCI bridges.

With just a handful of 8-port NICs, you can exceed the current 
slot-based limit on physical hardware.  It's not an extremely common 
configuration but it does exist.

BTW, I don't think it's possible to hot-add physical functions.  I 
believe I know of a card that supports dynamic add of physical functions 
(pre-dating SR-IOV).

Regards,

Anthony Liguori

> Daniel
>