On 09/07/2016 04:06 AM, Marcel Apfelbaum 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.
I'm just now writing the code that auto-adds *-ports as they are needed, and
doing it this way simplifies it *immensely*.
When I had to think about the possibility of needing upstream/downstream
switches, as an endpoint device was added, I would need to check if a
(root|downstream)-port was available and if not I might
be able to just add a root-port, or I might have to add a downstream-port; if
the only option was a downstream port, then *that* might require adding a new
*upstream* port.
If I can limit libvirt to only auto-adding root-ports (and if there is no
downside to putting multiple root ports on a single root bus port), then I just
need to find an empty function of an empty
slot on the root bus, add a root-port, and I'm done (and since 224 is *a lot*,
I think at least for now it's okay to punt once they get past that point).
So, *is* there any downside to doing this?