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From:   Doug Anderson <dianders@chromium.org>
Date:   Thu, 24 Jun 2021 06:46:50 -0700
Message-ID: <CAD=FV=VQEM=Gtzrkd-s_ufzi_Y7b1GPCOVROftmjLDWiMEd0qA@mail.gmail.com>
Subject: Re: [PATCH 3/6] PCI: Indicate that we want to force strict DMA for
 untrusted devices
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Will Deacon <will@kernel.org>,
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        Ulf Hansson <ulf.hansson@linaro.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
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        Rob Clark <robdclark@chromium.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
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        Roedel <joro@8bytes.org>," <iommu@lists.linux-foundation.org>,
        Sonny Rao <sonnyrao@chromium.org>,
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Hi,

On Thu, Jun 24, 2021 at 6:38 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Mon, Jun 21, 2021 at 04:52:45PM -0700, Douglas Anderson wrote:
> > At the moment the generic IOMMU framework reaches into the PCIe device
> > to check the "untrusted" state and uses this information to figure out
> > if it should be running the IOMMU in strict or non-strict mode. Let's
> > instead set the new boolean in "struct device" to indicate when we
> > want forced strictness.
> >
> > NOTE: we still continue to set the "untrusted" bit in PCIe since that
> > apparently is used for more than just IOMMU strictness. It probably
> > makes sense for a later patchset to clarify all of the other needs we
> > have for "untrusted" PCIe devices (perhaps add more booleans into the
> > "struct device") so we can fully eliminate the need for the IOMMU
> > framework to reach into a PCIe device.
>
> It feels like the iommu code should not be messing with pci devices at
> all, please don't do this.

I think it's generally agreed that having the IOMMU code reach into
the PCIe code is pretty non-ideal, but that's not something that my
patch series added. The IOMMU code already has special cases to reach
into PCIe devices to decide strictness. I was actually trying to
reduce the amount of it.

> Why does this matter?  Why wouldn't a pci device use "strict" iommu at
> all times?  What happens if it does not?  Why are PCI devices special?

This is something pre-existing in Linux. In my patch series I was
trying to make PCI devices less special and take some of the concepts
from there and expand them, but in a cleaner way. It sounds like in my
v2 I should steer away from this and leave the existing PCI hacks
alone.

-Doug