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Re: [Qemu-devel] Hitting 29 NIC limit


From: Anthony Liguori
Subject: Re: [Qemu-devel] Hitting 29 NIC limit
Date: Thu, 14 Oct 2010 07:59:00 -0500
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On 10/14/2010 07:36 AM, Markus Armbruster wrote:
Avi Kivity<address@hidden>  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.

multifunction=on sets the multifunction bit for the PCI device. Then it's a matter of setting the address to be a specific function.

Our default platform devices are actually multifunction.

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.

Yup.

Regards,

Anthony Liguori

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