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Date:   Tue, 13 Nov 2018 18:12:58 +0200
From:   Mika Westerberg <mika.westerberg@linux.intel.com>
To:     Yehezkel Bernat <yehezkelshb@gmail.com>
Cc:     iommu@lists.linux-foundation.org, joro@8bytes.org,
        David Woodhouse <dwmw2@infradead.org>,
        baolu.lu@linux.intel.com, ashok.raj@intel.com,
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        jacob.jun.pan@intel.com, Andreas Noever <andreas.noever@gmail.com>,
        michael.jamet@intel.com, lukas@wunner.de,
        Christian Kellner <ckellner@redhat.com>,
        Mario Limonciello <Mario.Limonciello@dell.com>,
        Anthony Wong <anthony.wong@canonical.com>,
        linux-acpi@vger.kernel.org, linux-pci@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/4] thunderbolt: Export IOMMU based DMA protection
 support to userspace
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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. ;-)