usb 3.0 half speed with pci=nomsi

[Skunk Worx <skunkworx@xxxxxxxxxxx>]

Hi,

I have two similar linux machines with legacy PCI / USB 3.0 cards 
plugged in. These use the NEC uPD720200 USB 3.0 Host Controller and the 
xhci module.

System A "Fast":
****************
USB 3.0 read performance of about 80 MB/s
doesn't need pci=nomsi to see the USB 3.0 devices

Snips of lspci -vv :
--------------------
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- 
Stepping- SERR+ FastB2B- DisINTx+

Interrupt: pin A routed to IRQ 21

Region 0: Memory at febfe000 (64-bit, non-prefetchable) [size=8K]

Capabilities: [90] MSI-X: Enable+ Count=8 Masked-
		
Capabilities: [a0] Express (v2) Endpoint, MSI 00
		DevSta:	CorrErr+ UncorrErr- FatalErr- UnsuppReq+ AuxPwr- TransPend-

Capabilities: [100] #1033

System B "Slow":
****************
USB 3.0 read performance of about 40 MB/s
needs pci=nomsi to see the USB 3.0 devices

Snips of lspci -vv :
--------------------
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- 
Stepping- SERR- FastB2B- DisINTx-

Interrupt: pin A routed to IRQ 17

Region 0: Memory at fb000000 (64-bit, non-prefetchable) [size=8K]

Capabilities: [90] MSI-X: Enable- Count=8 Masked-

Capabilities: [a0] Express (v2) Endpoint, MSI 00
		DevSta:	CorrErr- UncorrErr- FatalErr- UnsuppReq- A

Is it likely that the kernel option is what's dropping the USB 3.0 
performance to about 1/2 speed?

If there is a suggestion to up the speed please let me know.

---
John
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