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While trying to set up IOMMU, I came across a post that mentioned that you need not just software but also hardware support for it to work. The hardware support includes not just the CPU but also the motherboard/chipset.

Since you need VT-d for IOMMU, I checked the compatibility of my CPU (4790K) and motherboard and found something interesting which I can't explain.

My motherboard "GA-Z97X-UD3H" has the Z97 chipset which according to Intel's ark doesn't support VT-d, however the board offers me VT-d in the BIOS and its close brother, the "GA-Z97[X]-UD5H" is even listed as an IOMMU-supported motherboard on Wikipedia.

Now my question is, how is it possible that the chipset officially doesn't support VT-d but the motherboard still offers it? Can motherboard manufacturers add this kind of functionality themselves? Or what is going on?

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    The most likely explaination is that the intel ark page is simply incorrect. It could be that in the beginning there was no support, but it was later added and the page was not updated, or it was mistyped. Or perhaps its not the chipset but something else that provides VT-d support.
    – LPChip
    Commented May 6, 2017 at 11:17
  • VT-d isn't something a motherboard does or doesn't shop. The firmware might not have an option to enable or disable it. The difference is the motherboard is hardware but the firmware is software. VT-d support is based on the CPU (hardware) supporting it. Z97 is the top of the line chipset....so it does support VT-d provide us the specifications that say otherwise. The motherboard's firmware your asking about also support VT-d
    – Ramhound
    Commented May 6, 2017 at 13:24
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    I took at look at every single x97 chipset they all say they don't support VT-d but your CPU supports it. Logic dictates that if every chipset "doesn't" support for a specific CPU which specifically does support it, where that chipset was/is required to use that CPU One specification page is wrong
    – Ramhound
    Commented May 6, 2017 at 13:32
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    @Ramhound I would be very surprised to see Intel's ark to be incorrect, but given the evidence that is available, it might actually be the case.
    – comfreak
    Commented May 6, 2017 at 13:35

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As suggested in the comments, the simplest explanation is just that they never moved off from the "conventions" they had back when the precious memory controller was separate from the CPU, and they are just stating that "no, this chipset doesn't contain any enabling component in this regard".

Coupled with some other employees that just cannot manage to fix the miscomprehension.

Motherboard support is still somewhat needed (a broken BIOS can still ruin everything), but it should be no big deal. Intel itself is officially advertising this feature for their hardware and in datasheets.

Another theory for this mess was also that the "chipset VT-d status" is only actually being responsible for its own PCIe lanes, but it makes no sense considering the VT-d official specs puts DMA remapping before the southbridge.

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