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

From: Dmitry Osipenko
Date: Thu Sep 20 2018 - 13:09:36 EST

On 8/16/18 8:23 PM, Robin Murphy wrote:
On 15/08/18 20:56, Dmitry Osipenko wrote:
On Friday, 3 August 2018 18:43:41 MSK Robin Murphy wrote:
On 02/08/18 19:24, Dmitry Osipenko wrote:
On Friday, 27 July 2018 20:16:53 MSK Dmitry Osipenko wrote:
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)
{

ÂÂÂÂconst struct iommu_ops *ops = dev->bus->iommu_ops;
ÂÂÂÂstruct iommu_group *group;

ÂÂÂÂgroup = iommu_group_get(dev);
ÂÂÂÂif (!group)

ÂÂÂÂÂÂÂ return NULL;

ÂÂÂÂiommu_group_put(group);

ÂÂÂÂif (!ops->implicit_iommu_for_dma_is_allowed)

ÂÂÂÂÂÂÂ return true;

ÂÂÂÂreturn 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.

Guys, does it sound good to you or maybe you have something else on your
mind? Even if it's not an ideal solution, it fixes the immediate problem
and should be good enough for the starter.

To me that looks like a step ion the wrong direction that won't help at
all in actually addressing the underlying issues.

If the GPU driver wants to explicitly control IOMMU mappings instead of
relying on the IOMMU_DOMAIN_DMA abstraction, then it should use its own
unmanaged domain. At that point it shouldn't matter if a DMA ops domain
was allocated, since the GPU device will no longer be attached to it.

It is not obvious to me what solution you are proposing..

Are you saying that the detaching from the DMA IOMMU domain that is provided
by dma_ops() implementer (ARM32 arch for example) should be generalized and
hence there should be something like:

ÂÂÂÂdma_detach_device_from_iommu_dma_domain(dev);

that drivers will have to invoke.

No, I mean that drivers should not have to care at all. If the device has been given a set of DMA ops which rely on it being attached to a default DMA domain, that's not the driver's fault and it's not something the driver should have deal with. Either the DMA ops themselves should be robust and provide a non-IOMMU fallback if they detect that the device is currently attached to a different domain, or the attach operation (ideally in the IOMMU core, but at worst in the IOMMU driver's .attach_dev callback) should automatically tell the arch code to update the device's DMA ops appropriately for the target domain. There are already examples of both approaches dotted around arch-specific code, so the question is which particular solution is most appropriate to standardise on in what is intended to be generic code.
Okay, thank you for the clarification. It will be better to start with a workaround within the driver, maybe later we could come up with a universal solution. Is there any chance that you or Joerg could help with the standardization in the future? I don't feel that I have enough expertise and capacity to do that.

And hence there will be dma_map_ops.iommu_detach_device() that dma_ops()
provider will have to implement. Thereby provider will detach device from DMA
domain, destroy the domain and update the DMA ops of the device.

Yes, there may be some improvements to make like having unused domains
not consume hardware contexts, but that's internal to the relevant IOMMU
drivers. If moving in and out of DMA ops domains leaves the actual
dma_ops broken, that's already a problem between the IOMMU API and the
arch DMA code as I've mentioned before.

Furthermore, given what the example above is trying to do,
arch_setup_dma_ops() is way too late to do it - the default domain was
already set up in iommu_group_get_for_dev() when the IOMMU driver first
saw that device. An "opt-out" mechanism that doesn't actually opt out
and just bodges around being opted-in after the fact doesn't strike me
as something which can grow to be robust and maintainable.

For the case where a device has some special hardware relationship with
a particular IOMMU context, the IOMMU driver *has* to be completely
aware of that, i.e. it needs to be described in DT/ACPI, either via some
explicit binding or at least inferred from some SoC/instance-specific
IOMMU compatible. Then the IOMMU driver needs to know when the driver
for that device is requesting its special domain so that it provide the
correct context (and *not* allocate that context for other uses).
Anything which just relies on the order in which things currently happen
to be allocated is far too fragile long-term.

If hardware has some restrictions, then that should be reflected in the
hardware description. But that's not what we are trying to solve, at least
there is no such problem right now for NVIDIA Tegra.

OK, maybe I misunderstood "HW performs context switches and so particular allocations have to be assigned to a particular contexts IOMMU domain." - is it that the domain can be backed by any hardware context and the Tegra GPU driver only needs to know *which* one, rather then needing a specific hard-wired context to be allocated as in the Qcom case?

Yes, I can't recall that Tegra has any limitations like Qcom has.