On 09/07/2016 09:21 AM, Gerd Hoffmann wrote:
Hi,
ports, if that's allowed). For example:
- 1-32 ports needed: use root ports only
- 33-64 ports needed: use 31 root ports, and one switch with 2-32
downstream ports
I expect you rarely need any switches. You can go multifunction with
the pcie root ports. Which is how physical q35 works too btw, typically
the root ports are on slot 1c for intel chipsets:
nilsson root ~# lspci -s1c
00:1c.0 PCI bridge: Intel Corporation 7 Series/C210 Series Chipset
Family PCI Express Root Port 1 (rev c4)
00:1c.1 PCI bridge: Intel Corporation 7 Series/C210 Series Chipset
Family PCI Express Root Port 2 (rev c4)
00:1c.2 PCI bridge: Intel Corporation 7 Series/C210 Series Chipset
Family PCI Express Root Port 3 (rev c4)
Root bus has 32 slots, a few are taken (host bridge @ 00.0, lpc+sata @
1f.*, pci bridge @ 1e.0, maybe vga @ 01.0), leaving 28 free slots. With
8 functions each you can have up to 224 root ports without any switches,
and you have not many pci bus numbers left until you hit the 256 busses
limit ...
Good point, maybe libvirt can avoid adding switches unless the user explicitly
asked for them. I checked and it a actually works fine in QEMU.
BTW, when we will implement ARI we can have up to 256 Root Ports on a single
slot...