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[Qemu-devel] >256 Virtio-net-pci hotplug Devices


From: Kinsella, Ray
Subject: [Qemu-devel] >256 Virtio-net-pci hotplug Devices
Date: Mon, 17 Jul 2017 21:50:29 +0000

Hi folks,

I am trying to create a VM that supports hot-plugging a large number of 
virtio-net-pci device, 
up to 1000 devices initially.

>From the docs (see below) and from playing with QEMU, it looks like there are 
>two options.
Both with limitations.

PCI Express switch

It looks like using a PCI Express switch hierarchy is not an option due to bus 
exhaustion. 
Each downstream port creates a separate bus and each downstream port only 
supports hot-plugging a single device.
So this gives us a max of 256-ish buses/devices pairs.

PCI Root Ports

The other option is use a flatter hierarchy, with a number of multi-function 
PCI Root Ports hanging off 'pcie.0'.
However each 'PCI Root Port' can support hot-plugging a single device. 
So this method really becomes a function of how many free address we have on 
'pci.0'.
If we make room for say 16 multifunction devices, we get 16*8 ... 128
So ultimately, this will approach will give us a similar to number to using a 
switch. 

Is there another method?
( pxb-pcie doesn't support hotplug for instance, and only a single pcie domain 
is supported qemu )

Thanks,

Ray K

Pcie.txt, 
( https://github.com/qemu/qemu/blob/master/docs/pcie.txt )
Q35 preso
( http://wiki.qemu.org/images/4/4e/Q35.pdf )

 





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