Re: [kvm-devel] [PATCH 3/3] virtio PCI device

Arnd Bergmann
2007-11-27T09:27:33Z

On Tuesday 27 November 2007, Avi Kivity wrote:
> > :-)  Do you know if there is a hard limit on the number of devices on
> > a PCI bus?  My concern was that it was limited by something stupid
> > like an 8-bit identifier.
>
> IIRC pci slots are 8-bit, but you can have multiple buses, so
> effectively 16 bits of device address space (discounting functions which
> are likely not hot-pluggable).

You have an 8 bit bus number and an 8 bit device/function number.
The function number is 3 bits, so if you want to use only function 0
for everything, you are limited to a little under 8192 (2^(8+5)) devices
per PCI domain. PC style hardware cannot easily address multiple PCI
domains, but I think you can have them if you assume that the guest is
using mmconfig.

For using multiple buses, the easiest way could be to have every
device/function on bus 0 be a bridge by itself, so you end up with a
flat number space for the actual devices,

$ lspci -t
[0000:00]-+-00.0-[0000:01]--+-00.0
| +-01.0
| +-02.0
| + ...
| \-3f.0
+-00.1-[0000:02]--+-00.0
| +-01.0
| +-02.0
| + ...
| \-3f.0
+ ...
|
+-3f.6-[0000:ff]--+-00.0
+-01.0
+-02.0
+ ...
\-3f.0