Re: pcie-root-port limits for q35?

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Oct 08, 2024 at 03:44:33PM +0530, Tejus GK wrote:
> Hi, recently I was trying to play with the PCIe limits on the q35 machine
> type. I see that docs/pcie.txt mentions:
> 
> > The IO space is very limited, to 65536 byte-wide IO ports, and may
> > even be fragmented by fixed IO ports owned by platform devices
> > resulting in at most 10 PCI Express Root Ports or PCI Express
> > Downstream Ports per system if devices with IO BARs are used in the
> > PCI Express hierarchy
> 
> But in my own experimental setup, I was able to successfully create and
> power on a VM with 232 pcie-root-ports, with each root port hosting a
> virtio-net device on it. The root ports, and the network devices end up
> appearing inside the VM's PCI enumeration as well. So I'm left wondering
> where this limit exactly come from? And I'm not sure if I am missing
> something here, since I'm able to go way above the limit mentioned above.

Note the very last part of the text you quoted:

   ..if devices with IO BARs are used..

     ^^

AFAIK, the virtio devices (at least in modern mode) don't use IO BARs,
thus avoiding the mentioned limit.

With regards,
Daniel
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