On Wed, 7 Sep 2016 11:06:45 +0300 Marcel Apfelbaum <mar...@redhat.com> wrote:
> 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... "Root Ports on a single slot"... The entire idea of ARI is that there is no slot, the slot/function address space is combined into one big 8-bit free-for-all. Besides, can you do ARI on the root complex? Typically you need to look at whether the port upstream of a given device supports ARI to enable, there's no upstream PCI device on the root complex. This is the suggestion I gave you for switches, if the upstream switch port supports ARI then we can have 256 downstream switch ports (assuming ARI isn't only specific to downstream ports). Thanks, Alex
