[vfio-users] z370 iommu groupings

[Nick S]

I am looking to build a second server capable of running multiple virtual
machines with physical GPU assignment and I am wondering whether Gen 8
Intel CPU would be a feasible choice. I am looking for something with 6 or
more cores and Intel graphics for the host. Gen 8 seems to be the only
available choice so far with not a single >4core xeon having IGD. Due to
the requirement to run multiple VMs I apparently need ACS capabilities.

Here is what I found so far:

1) i7-8700 does not have ACS registers in its datasheet
2) associated with it z370 chipset apparently has them

If I understand the implications of this right, the IOMMU grouping will be
OK if I end up with up to 1 PCIE slot handled by CPU and others handled by
PCH. My problem is that I have no idea how to figure out this assignment
for a motherboard without actually buying it, installing linux and running
the relevant commands. I found just one Gen 8 success story for Asrock
z370m pro4 where author claims that each slot he tried got its own group.
Unfortunately is this motherboard is micro ATX and I am looking for a
regular size motherboard with 4 16x slots adequately spaced so that each
can hold an individual 2 slot thick GPU. I found a couple of Extended ATX
options that seem to be ideal for me, but I have no idea how to predict
grouping.
I.E. this one has all the slots I need:
https://www.newegg.com/Product/Product.aspx?Item=N82E16813119031

And it claims to support 8x/4x/4x combination on the first 3 16x slots. It
does feel like CPU's 16 lanes may be getting spread over those 3 slots with
4th one being handled by the chipset. Does this mean that I won't be able
to separately assign card from first 3 slots? Anybody has other z370 / Gen
8 success stories with IOMMU groups?

Thank you
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