Re: [RFC PATCH v1 0/6] Resolve unwanted DMA backing with IOMMU

[Jordan Crouse <jcrouse@codeaurora.org>]

On Fri, Jul 27, 2018 at 05:02:37PM +0100, Robin Murphy wrote:
> On 27/07/18 15:10, Dmitry Osipenko wrote:
> >On Friday, 27 July 2018 12:03:28 MSK Will Deacon wrote:
> >>On Fri, Jul 27, 2018 at 10:25:13AM +0200, Joerg Roedel wrote:
> >>>On Fri, Jul 27, 2018 at 02:16:18AM +0300, Dmitry Osipenko wrote:
> >>>>The proposed solution adds a new option to the base device driver
> >>>>structure that allows device drivers to explicitly convey to the drivers
> >>>>core that the implicit IOMMU backing for devices must not happen.
> >>>
> >>>Why is IOMMU mapping a problem for the Tegra GPU driver?
> >>>
> >>>If we add something like this then it should not be the choice of the
> >>>device driver, but of the user and/or the firmware.
> >>
> >>Agreed, and it would still need somebody to configure an identity domain so
> >>that transactions aren't aborted immediately. We currently allow the
> >>identity domain to be used by default via a command-line option, so I guess
> >>we'd need a way for firmware to request that on a per-device basis.
> >
> >The IOMMU mapping itself is not a problem, the problem is the management of
> >the IOMMU. For Tegra we don't want anything to intrude into the IOMMU
> >activities because:
> >
> >1) GPU HW require additional configuration for the IOMMU usage and dumb
> >mapping of the allocations simply doesn't work.
> 
> Generally, that's already handled by the DRM drivers allocating
> their own unmanaged domains. The only problem we really need to
> solve in that regard is that currently the device DMA ops don't get
> updated when moving away from the managed domain. That's been OK for
> the VFIO case where the device is bound to a different driver which
> we know won't make any explicit DMA API calls, but for the more
> general case of IOMMU-aware drivers we could certainly do with a bit
> of cooperation between the IOMMU API, DMA API, and arch code to
> update the DMA ops dynamically to cope with intermediate subsystems
> making DMA API calls on behalf of devices they don't know the
> intimate details of.
> 
> >2) Older Tegra generations have a limited resource and capabilities in regards
> >to IOMMU usage, allocating IOMMU domain per-device is just impossible for
> >example.
> >
> >3) HW performs context switches and so particular allocations have to be
> >assigned to a particular contexts IOMMU domain.
> 
> I understand Qualcomm SoCs have a similar thing too, and AFAICS that
> case just doesn't fit into the current API model at all. We need the
> IOMMU driver to somehow know about the specific details of which
> devices have magic associations with specific contexts, and we
> almost certainly need a more expressive interface than
> iommu_domain_alloc() to have any hope of reliable results.

This is correct for Qualcomm GPUs - The GPU hardware context switching 
requires a specific context and there are some restrictions around
secure contexts as well.

We don't really care if the DMA attaches to a context just as long as it
doesn't attach to the one(s) we care about. Perhaps a "valid context" mask would
work in from the DT or the device struct to give the subsystems a clue as to
which domains they were allowed to use. I recognize that there isn't a
one-size-fits-all solution to this problem so I'm open to different ideas.

Jordan

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