IOMMU groups: better with Intel root ports, or with PEX8606 switch?

[Dana Goyette <DanaGoyette@gmail.com>]

I have purchased both a SuperMicro X10SAE and an X10SAT, and I need to 
soon decide which one to keep.

The SuperMicro X10SAT has all the PCIe x1 slots hidden behind a PLX 
PEX8066 switch, which claims to support ACS.  I'd expect the devices 
downstream of the PLX switch to be in separate groups.

With Linux 3.13-rc5 and "enable overrides for missing ACS capabilities" 
applied and set for the Intel root ports, the devices behind the switch 
remain stuck in the same group.

In terms of passing devices to different VMs, which is better: all 
devices on different root ports, or all devices behind the one 
ACS-supporting switch?

X10SAT IOMMU groups:

### Group 0 ###
     00:00.0 Host bridge: Intel Corporation Xeon E3-1200 v3 Processor 
DRAM Controller (rev 06)

### Group 1 ###
     00:01.0 PCI bridge: Intel Corporation Xeon E3-1200 v3/4th Gen Core 
Processor PCI Express x16 Controller (rev 06)
     01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. 
[AMD/ATI] Juniper PRO [Radeon HD 5750]
     01:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] 
Juniper HDMI Audio [Radeon HD 5700 Series]

### Group 2 ###
     00:02.0 VGA compatible controller: Intel Corporation Xeon E3-1200 
v3 Processor Integrated Graphics Controller (rev 06)

### Group 3 ###
     00:03.0 Audio device: Intel Corporation Xeon E3-1200 v3/4th Gen 
Core Processor HD Audio Controller (rev 06)

### Group 4 ###
     00:14.0 USB controller: Intel Corporation 8 Series/C220 Series 
Chipset Family USB xHCI (rev 05)

### Group 5 ###
     00:16.0 Communication controller: Intel Corporation 8 Series/C220 
Series Chipset Family MEI Controller #1 (rev 04)
     00:16.3 Serial controller: Intel Corporation 8 Series/C220 Series 
Chipset Family KT Controller (rev 04)

### Group 6 ###
     00:19.0 Ethernet controller: Intel Corporation Ethernet Connection 
I217-LM (rev 05)

### Group 7 ###
     00:1a.0 USB controller: Intel Corporation 8 Series/C220 Series 
Chipset Family USB EHCI #2 (rev 05)
### Group 8 ###
     00:1b.0 Audio device: Intel Corporation 8 Series/C220 Series 
Chipset High Definition Audio Controller (rev 05)

### Group 9 ###
     00:1c.0 PCI bridge: Intel Corporation 8 Series/C220 Series Chipset 
Family PCI Express Root Port #1 (rev d5)

### Group 10 ###
     00:1c.1 PCI bridge: Intel Corporation 8 Series/C220 Series Chipset 
Family PCI Express Root Port #2 (rev d5)
     03:00.0 PCI bridge: PLX Technology, Inc. PEX 8606 6 Lane, 6 Port 
PCI Express Gen 2 (5.0 GT/s) Switch (rev ba)
     04:01.0 PCI bridge: PLX Technology, Inc. PEX 8606 6 Lane, 6 Port 
PCI Express Gen 2 (5.0 GT/s) Switch (rev ba)
     04:04.0 PCI bridge: PLX Technology, Inc. PEX 8606 6 Lane, 6 Port 
PCI Express Gen 2 (5.0 GT/s) Switch (rev ba)
     04:05.0 PCI bridge: PLX Technology, Inc. PEX 8606 6 Lane, 6 Port 
PCI Express Gen 2 (5.0 GT/s) Switch (rev ba)
     04:07.0 PCI bridge: PLX Technology, Inc. PEX 8606 6 Lane, 6 Port 
PCI Express Gen 2 (5.0 GT/s) Switch (rev ba)
     04:09.0 PCI bridge: PLX Technology, Inc. PEX 8606 6 Lane, 6 Port 
PCI Express Gen 2 (5.0 GT/s) Switch (rev ba)
     08:00.0 USB controller: Renesas Technology Corp. uPD720201 USB 3.0 
Host Controller (rev 03)
     09:00.0 PCI bridge: Texas Instruments XIO2213A/B/XIO2221 PCI 
Express to PCI Bridge [Cheetah Express] (rev 01)
     0a:00.0 FireWire (IEEE 1394): Texas Instruments XIO2213A/B/XIO2221 
IEEE-1394b OHCI Controller [Cheetah Express] (rev 01)

### Group 11 ###
     00:1c.3 PCI bridge: Intel Corporation 8 Series/C220 Series Chipset 
Family PCI Express Root Port #4 (rev d5)
### Group 12 ###
     00:1c.4 PCI bridge: Intel Corporation 8 Series/C220 Series Chipset 
Family PCI Express Root Port #5 (rev d5)

### Group 13 ###
     00:1d.0 USB controller: Intel Corporation 8 Series/C220 Series 
Chipset Family USB EHCI #1 (rev 05)

### Group 14 ###
     00:1f.0 ISA bridge: Intel Corporation C226 Series Chipset Family 
Server Advanced SKU LPC Controller (rev 05)
     00:1f.2 SATA controller: Intel Corporation 8 Series/C220 Series 
Chipset Family 6-port SATA Controller 1 [AHCI mode] (rev 05)
     00:1f.3 SMBus: Intel Corporation 8 Series/C220 Series Chipset 
Family SMBus Controller (rev 05)
     00:1f.6 Signal processing controller: Intel Corporation 8 Series 
Chipset Family Thermal Management Controller (rev 05)

### Group 15 ###
     02:00.0 SATA controller: ASMedia Technology Inc. ASM1062 Serial ATA 
Controller (rev 01)

### Group 16 ###
     0b:00.0 Ethernet controller: Intel Corporation I210 Gigabit Network 
Connection (rev 03)