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Z490 motherboard question regarding VFIO options

Hello,

I am planning a build with the new 10th gen Intel processors and the Z490 chipset. I would like to be able to setup as many as 3 Windows guests, meaning up to 3 GPUs passed through to guest VMs running Windows. I know that AMD Ryzen 3xxx and TR would likely be better for this with the extra cores (for both Ryzen and TR) and PCIe lanes on TR, but for other reasons, I am going Intel on this one.

First question: My understanding is that with integrated graphics, e.g. i7-10700K with UHD 630, I can use that for the Linux host, and as long as I have 3 PCIe x16 slots (and they are each in their own IOMMU group), I can pass each of them to individual Windows guest VMs. Is that correct?

Second question: Ideally, the PCIe lanes used for the GPUs should be attached to the CPU, to avoid the bottleneck of having to go through the DMI from the chipset, right?

Third question: (assuming answer to question #2 is “yes”) Looking at the Z490 motherboards that will be available with 3+ PCIe x16 slots connected to CPU, I see the Asus ROG STRIX Z490-H GAMING and Supermicro C9Z490-PG. Based on the spec sheets, I believe the Asus is bifurcating the CPU lanes into x8, x4, and x4, whereas the Supermicro is using a PLX chip to multiplex into x8, x8, x8, x8. Any thoughts as to which board would work better for my application? Is it more difficult to passthrough with a PLX chip? If x4 is enough for GPU, then I could avoid the extra cost of the PLX chip.

Thanks!

I can answer your first question. yes, 3 PCI slots, each in its own IOMMU group means you can pass it to 3 different vms

I don’t know how bad it is to pass GPU stream through plex chip but as far as I know it’s virtually passing your GPU through chipset. As in Plex chip adds latency while bifurcation is just lane splitting so there are no compnents in between. For gaming it’s always recommended to use direct lanes and not chipset ones to avoid said latency and bottlenecks. Also Plex chips don’t magically add bandwidth. You will still get the same lane bandwidth but the beauty of Plex chip is that if say one graphics card idles then others will get more bandwidth available while bifurcation doesn’t do that. Though like I said I don’t know how bad plex chip latency is and also depends on how good of Plex chip used, some have low latency some have high. I would think that if Asus and other “gaming” mobo manufactures didn’t see plex chip as bad thing they would have used one long time ago at least in their high tier mobos. Server manufacturer obviously doesn’t care about latency, it’s all about bandwidth and expansions.