[Xen-users] VT-d superpages (2MB, 1GB) and the intel iGPU

[Ben Himberg <bhimberg@xxxxxxxxx>]

So I have a Tyan c226 with a 1246v3 Xeon. I noticed many users have something like this in their xl dmesg:

(XEN) Intel VT-d iommu 0 supported page sizes: 4kB.
(XEN) Intel VT-d iommu 1 supported page sizes: 4kB, 2MB, 1GB.

Now as I understand it VT-d for this processor/chipset has two iommu engines: 1 for the GPU, 1 for the rest of the system. When the iGPU is enabled the specification states that the default engine may 'should' disable large page support and look like this:

(XEN) Intel VT-d iommu 0 supported page sizes: 4kB.
(XEN) Intel VT-d iommu 1 supported page sizes: 4kB.

Which is exactly what I get with my board. Furthermore, shared EPT, snoop control are also disabled when the iGPU is enabled (basically the bios/xen default to the common standards between the two iommu engines).

Kudos to Tyan for implementing their bios properly. The question though: why do many of you seem to have both engines enabled AND have large pages enabled on the default (listed second) engine? When I disable my iGPU I get the following:

(XEN) Intel VT-d iommu 0 supported page sizes: 4kB, 2MB, 1GB.
(XEN) Intel VT-d Snoop Control enabled.
(XEN) Intel VT-d Dom0 DMA Passthrough not enabled.
(XEN) Intel VT-d Queued Invalidation enabled.
(XEN) Intel VT-d Interrupt Remapping enabled.
(XEN) Intel VT-d Shared EPT tables enabled.

And when I enable it:
(XEN) Intel VT-d iommu 0 supported page sizes: 4kB.
(XEN) Intel VT-d iommu 1 supported page sizes: 4kB.
(XEN) Intel VT-d Snoop Control not enabled.
(XEN) Intel VT-d Dom0 DMA Passthrough not enabled.
(XEN) Intel VT-d Queued Invalidation enabled.
(XEN) Intel VT-d Interrupt Remapping enabled.
(XEN) Intel VT-d Shared EPT tables not enabled.

My understanding is that large pages are important for PCIE transfers, and I'm building this box to basically run an OpenCL headless server for my simulations. Is there any way I can enable large page sizes on the default (second) iommu engine AND keep the iGPU enabled?

For reference, check page 46 of the following for intels thoughts:
http://www.intel.com/content/dam/www/public/us/en/documents/datasheets/xeon-e3-1200v3-vol-1-datasheet.pdf

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