Security and functional implications of iommu=no-igfx (helps me with video tearing, oddly)

[tonsimple]

Qubes OS version (e.g., R3.2):

3.2

Affected TemplateVMs (e.g., fedora-23, if applicable):

dom0?

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Expected behavior:

Option "TearFree" "true" in xorg.conf would remove tearing without iommu=no-igfx being passed as boot parameter

Actual behavior:

Tearing only seems to disappear for me after iommu=no-igfx is passed and that option is on
Also, seems iommu=no-igfx generally improves my video's (Integrated graphics, skylake Core i7 + ASUS Hero VIII) perfomance

Steps to reproduce the behavior:

Have similar hardware (I strongly suspect it's hardware-dependent, have no issues enabling TearFree w/o extra iommu=no-igfx fuss on a laptop)
Try setting Option TearFree to true without passing iommu=no-igfx as boot parameter
reboot
Try setting Option TearFree to true WITH passing iommu=no-igfx as boot parameter
reboot

General notes:

Not so much an issue (I am almost satisfied with how my video is working for qubes now) but I'd like to understand better what iommu=no-igfx implies and whether it's worth the (minor, but I think non-placebo) improvement of video behavior.

Do I understand correctly that IOMMU/vt-d is still working okay-dokay for everything that is not the integrated GPU?

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Related issues:

[andrewdavidwong]

Not so much an issue (I am almost satisfied with how my video is working for qubes now) but I'd like to understand better what iommu=no-igfx implies and whether it's worth the (minor, but I think non-placebo) improvement of video behavior.

This would be more appropriate for qubes-devel. We try to reserve qubes-issues for things that are actionable.

[andrewdavidwong]

Not so much an issue (I am almost satisfied with how my video is working for qubes now) but I'd like to understand better what iommu=no-igfx implies and whether it's worth the (minor, but I think non-placebo) improvement of video behavior.

This would be more appropriate for qubes-devel. We try to reserve qubes-issues for things that are actionable.