linux-kernel - Re: [PATCH 3/6] PCI: Indicate that we want to force strict DMA for untrusted devices

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Date:   Thu, 24 Jun 2021 15:38:18 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     rafael@...nel.org, rafael.j.wysocki@...el.com, will@...nel.org,
        robin.murphy@....com, joro@...tes.org, bjorn.andersson@...aro.org,
        ulf.hansson@...aro.org, adrian.hunter@...el.com,
        bhelgaas@...gle.com, robdclark@...omium.org,
        linux-arm-msm@...r.kernel.org, linux-pci@...r.kernel.org,
        quic_c_gdjako@...cinc.com, iommu@...ts.linux-foundation.org,
        sonnyrao@...omium.org, saiprakash.ranjan@...eaurora.org,
        linux-mmc@...r.kernel.org, vbadigan@...eaurora.org,
        rajatja@...gle.com, saravanak@...gle.com, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] PCI: Indicate that we want to force strict DMA for
 untrusted devices

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.

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?

greg k-h