pcie-root-port limits for q35?

pcie-root-port limits for q35?

[Tejus GK]

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.


regards,
tejus

Re: pcie-root-port limits for q35?

[Daniel P. Berrangé]

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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