xen 4.8.2 iommu=off problems

xen 4.8.2 iommu=off problems

[James Dingwall]

Hi,

We have 3x HPE DL180 Gen9 servers, one of these is dual CPU the others single.  They are all running the same 
xen 4.8.2 build (plus some XSA patches) and Linux 4.1.46 dom0/guest kernel.  On the single CPU systems we can 
successfully passthrough ports from the onboard controller (igb) and additional PCIe card when iommu=off 
x2apic=off (vs iommu=on), on the dual cpu system the onboard interfaces appear but on passthrough to the driver 
domain the kernel continually logs "igb 0000:05:00.0 eth0: Reset adapter" messages for both interfaces and they 
cannot be used.

I have compared firmware/BIOS versions on all the systems and found them to be identical (although not 
necessarily latest).  Previously all the systems ran xen 4.4.3 + Linux 4.1.43 without any passthrough problems 
and iommu=off.

The combination of iommu=off and x2apic=on does not work as xen panics that it "Couldn't enable IOMMU" (I think 
from xen/drivers/passthrough/iommu.c).  The reason for using iommu=off is that we have noted a performance drop 
in HVM guests for iommu=on.

I'm looking for some suggestions on how to proceed in order to get the dual cpu system working again with 
iommu=off.

Thanks,
James

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