Re: [PATCH 4/4] thunderbolt: Export IOMMU based DMA protection support to userspace

[Mika Westerberg <mika.westerberg-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Tue, Nov 13, 2018 at 05:38:53PM +0200, Yehezkel Bernat wrote:
> Good point. But I thought about per-TBT-device decision. If the platform is
> configured for IOMMU+"user" security level, while approving the device the user
> may want to set also in which IOMMU group to put all the PCIe devices connected
> to it. The same goes if kernel is supposed to auto-approve such devices based on
> an internal table. The point is that we can think on a configuration where the
> devices aren't tunneled yet and the decision about IOMMU can still be changed.

Right, some of these systems have security level set to "user" so there
we could have a way to put the device into passthrough mode before it
appears on the PCIe bus. That would require some sort of API on the
IOMMU side, though.

> As you mentioned this isn't the common configuration anyway, so it probably
> doesn't worth all this hassle.

AFAIK mixing the two is not something they are going to be supporting in
Windows so I would not expect it to be common. I think the ultimate goal
is to move away from security levels towards IOMMU DMA protection so in
future I would expect more and more systems with IOMMU enabled +
security level set to "none".

So I agree with you that it probably is not worth doing at least without
having more data about real performance issues around this. ;-)