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

[Dmitry Osipenko <digetx-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Friday, 27 July 2018 20:03:26 MSK Jordan Crouse wrote:
> 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.

Designating whether implicit IOMMU backing is appropriate for a device via 
device-tree property sounds a bit awkward because that will be a kinda 
software description (of a custom Linux driver model), while device-tree is 
supposed to describe HW.

What about to grant IOMMU drivers with ability to decide whether the implicit 
backing for a device is appropriate? Like this:

bool implicit_iommu_for_dma_is_allowed(struct device *dev)
{
	const struct iommu_ops *ops = dev->bus->iommu_ops;
	struct iommu_group *group;

	group = iommu_group_get(dev);
	if (!group)
		return NULL;

	iommu_group_put(group);

	if (!ops->implicit_iommu_for_dma_is_allowed)
		return true;

	return ops->implicit_iommu_for_dma_is_allowed(dev);
}

Then arch_setup_dma_ops() could have a clue whether implicit IOMMU backing for 
a device is appropriate.