Re: [RFC PATCH] iommu: arm-smmu-nvidia: Add default domain type implementation op

Thierry Reding
2023-07-14T06:56:56Z

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On Fri, Jul 14, 2023 at 11:14:08AM +0800, Baolu Lu wrote:
> On 2023/7/13 21:31, Thierry Reding wrote:
> > On Tue, Jul 11, 2023 at 05:55:11PM +0200, Thierry Reding wrote:
> > > On Tue, Jul 11, 2023 at 01:58:34PM +0300, Stanimir Varbanov wrote:
> > > > Hi Thierry,
> > > >
> > > > Thank you for the comments!
> > > >
> > > > On 7/10/23 13:40, Thierry Reding wrote:
> > > > > On Mon, Jul 10, 2023 at 11:22:52AM +0300, Stanimir Varbanov wrote:
> > > > > > Add def_domain_type implementation op and override default IOMMU
> > > > > > domain Kconfig option (CONFIG_IOMMU_DEFAULT_PASSTHROUGH=y), which
> > > > > > could be enabled on some distros. The current quirk has been done
> > > > > > for Tegra234 machine, because I found the issue on it. The issue
> > > > > > itself appears on USB host controller which cannot be initialized
> > > > > > without IOMMU translation. Something more, we proved that IOMMU
> > > > > > translation is needed for display and GPU drivers as well.
> > > > > >
> > > > > > I evaluated few possible options to solve that:
> > > > > >
> > > > > > a) select default IOMMU domain from .def_domain_type op
> > > > > > b) Unset CONFIG_IOMMU_DEFAULT_PASSTHROUGH=n
> > > > > > c) add iommu.passthrough=0 on the kernel cmdline
> > > > > > d) firmware - ACPI / DT
> > > > > >
> > > > > > a) This option is implemented in the proposed patch.
> > > > > >
> > > > > > b) Since that the community has agreed that pass-through is preferred
> > > > > > as a default IOMMU domain option because this will avoid performance
> > > > > > impacts on some of the platforms [1]. On the other side we have examples
> > > > > > where you cannot even install Linux distribution on a machine where the
> > > > > > storage media cannot be detected and the system just hangs.
> > > > >
> > > > > That's not how I read that thread. It sounds more to me like Will and
> > > > > Robin had ideas on how to improve the performance and were planning to
> > > > > address these issues. It doesn't exactly sound to me like there was
> > > > > concensus to make passthrough the default.
> > > > >
> > > > > Having said that, given that it's possible for distributions and users
> > > > > to set CONFIG_IOMMU_DEFAULT_PASSTHROUGH=y, I think it would be useful in
> > > > > general to have a way of enforcing IOMMU translations if it's needed by
> > > > > the hardware.
> > > >
> > > > Exactly, the problem is that some platforms prefer passthrough to avoid
> > > > performance impacts but others cannot even boot the kernel (and thus
> > > > installation failure). Passing iommu.passthrough=0 should be an
> > > > administrator decision, balancing between security and performance.
> > > >
> > > > On the other hand the aforementioned mail thread gave some performance
> > > > numbers which might be are outdated having the improvements made in smmu
> > > > driver in mind. Unfortunately, I cannot confirm that the performance has
> > > > been improved during that time.
> > > >
> > > > >
> > > > > I'm not sure I fully understand the particular problems that you're
> > > > > seeing on Tegra234, though. I'm not aware of anything in the USB host
> > > > > controller driver (or hardware, for that matter) that would require the
> > > > > IOMMU to be enabled. The only peculiarity that I can think of is the
> > > > > firmware, which is typically loaded by an early bootloader and therefore
> > > > > might perhaps need the IOMMU to properly map this in the kernel.
> > > > > However, my understanding is that this firmware is loaded into special
> > > > > carveout regions which don't require remapping.
> > > >
> > > > On Jetson Orin AGX (R35.2.1) I see these errors:
> > > >
> > > > tegra-mc 2c00000.memory-controller: unknown: write @0x0000000000000080:
> > > > EMEM address decode error (EMEM decode error)
> > > >
> > > > tegra-xusb 3610000.usb: Error while assigning device slot ID
> > > > tegra-xusb 3610000.usb: Max number of devices this xHCI host supports is 36.
> > > > usb usb2-port3: couldn't allocate usb_device
> > > > tegra-mc 2c00000.memory-controller: unknown: write @0x0000000000000090:
> > > > EMEM address decode error (EMEM decode error)
> > > > tegra-xusb 3610000.usb: Error while assigning device slot ID
> > > > tegra-xusb 3610000.usb: Max number of devices this xHCI host supports is 36.
> > > > usb usb1-port3: couldn't allocate usb_device
> > > >
> > > > tegra-mc 2c00000.memory-controller: unknown: write @0x00000000000000a0:
> > > > EMEM address decode error (EMEM decode error)
> > > > tegra-xusb 3610000.usb: Error while assigning device slot ID
> > > > tegra-xusb 3610000.usb: Max number of devices this xHCI host supports is 36.
> > > > usb usb1-port4: couldn't allocate usb_device
> > > >
> > > > >
> > > > > However, passthrough is admittedly not something that we've thoroughly
> > > > > tested, so it's possible you're running into a use-case that I'm not
> > > > > aware of. In that case, could you provide a few more specifics (such as
> > > > > the DTB and .config) of your build configuration so that I can try and
> > > > > reproduce?
> > > >
> > > > To reproduce you have to add iommu.passthrough=1 on kernel cmdline. The
> > > > dtb is from Jetpack.
> > >
> > > I was able to reproduce this on Jetson Orin NX (the differences to AGX
> > > Orin should be negligible in this context), though I ended up patching
> > > the DTB to disable all SMMUs. What fixed it for me was to drop the
> > > dma-coherent property from the usb@3610000 node. Can you try that on
> > > your end to see if that works for you as well?
> > >
> > > Not that that's a proper solution, of course, but just trying to find
> > > out if there's perhaps something else going on.
> > >
> > > From the looks of it, it seems like these devices aren't actually DMA
> > > coherent inherently, but rather the SMMU translations make the accesses
> > > to memory coherent. I'm trying to find out the exact details, but if it
> > > turns out to be the case, then what we really want is a way for an IOMMU
> > > to mark any devices that get attached to it as DMA coherent. It's not
> > > sufficient to hard-code this in DT because there are various ways in
> > > which device can end up not attached to an IOMMU despite what the DT
> > > says.
> > >
> > > Jason, or anyone of the IOMMU folks, any thoughts on how this could be
> > > achieved? DT already has a way of walking up the "DMA hierarchy" looking
> > > for a DMA coherent parent, but again, making this rely entirely on DT
> > > seems insufficient.
> >
> > I've got a bit more information on what's happening here. There are
> > three different ways that a device's memory accesses can coherent on
> > Tegra: 1) when translated through the ARM SMMU, 2) some devices can
> > force coherency through configuration registers and 3) each device can
> > be forced to be coherent via the memory controller.
> >
> > Option 3) is not something we have much control over because this is
> > configured during early boot and the corresponding registers locked
> > down, so the OS can at maximum read out the configuration.
> >
> > Option 1) is what is typically used, so a common pattern is that if we
> > enable IOMMU translations via DT, we also set dma-coherent. Conversely,
> > if IOMMU translations are disabled via DT, the dma-coherent property
> > should also be removed because by default most devices will not be
> > hardcoded to be DMA coherent via option 3). Most device drivers will
> > also not program the device's configuration registers.
> >
> > As a result the desired configuration is to always enable SMMU and rely
> > on the SMMU translations to provide DMA coherency. As we've seen this
> > can be problematic, because the device tree doesn't always tell the true
> > story. For example even if the iommus property exists, the device may
> > not end up attached to the IOMMU for a number of reasons (the IOMMU
> > could itself be disabled, a kernel command-line option could prevent the
> > attachment, or a device could even be detached explicitly).
> >
> > So I think what we want to avoid is to mark all device tree nodes as
> > dma-coherent because it can lead to inconsistencies. A better option
> > would be to have this property inherited via the IOMMU if the IOMMU
> > translations themselves cause the coherency to be established. Now it
> > seems like DT already contains a way of doing that via the "DMA parent".
> > This works by looking up a special interconnects path called "dma-mem".
> > We already use this on Tegra to make the memory controller the DMA
> > parent of all memory clients (i.e. all DMA capable hardware blocks) in
> > order to enforce a bus-level DMA mask.
> >
> > However, in order for the DMA parent mechanism to work for IOMMU, we'd
> > need to redirect the DMA parent to be the IOMMU, but in that case we
> > loose the link to the memory controller. Unless, perhaps, if there's a
> > way to construct an ICC path from device to IOMMU and then to memory
> > controller and external memory controller (EMC).
> >
> > For reference, here's roughly what the data path looks like on Tegra:
> >
> > device --> MC --> SMMU enabled --> SMMU --> EMC --> DRAM
> > | ^
> > --> SMMU bypass -----------|
> >
> > SMMU can be enabled/disabled per device via a stream ID override
> > register in the memory controller.
> >
> > The biggest downside of that mechanism is still that it's a static
> > configuration and doesn't respect the actual runtime attachment of a
> > device to an IOMMU.
> >
> > Adding the DT folks to see if they have any good ideas on how best to
> > represent this from a DT point of view. Would it perhaps be an option
> > to consider the iommus property when walking up the DMA ancestry?
>
> Is it possible to handle this dynamically in the code? Say, set device
> to be DMA coherent in probe_finalize callback of Tegra iommu driver if
> the IOMMU translation for this device is on. And clear it in the iommu
> release device path.
>
> Normally we switch the DMA ops in the probe_finalize callback and iommu
> device release.

Yeah, I had looked into this as well. Intel, AMD and VirtIO all seem to
do this during .probe_finalize(), whereas on ARM64 this happens as part
of the bus' .dma_configure() callback.

One thing that we could potentially do is fiddle with the struct device
.dma_coherent member in .probe_finalize() and .release_device(), but I'm
not sure about the potential ramifications. That is, do we have places
in the code that assume dev->dma_coherent to be statically set during
device instantiation?

We would have to default to not marking devices as dma-coherent in DT
for that to work, though, because otherwise if someone were to disable
the IOMMU altogether, .probe_finalize() and .release_device() would
never get called and we'd never get a chance to override.

I wonder if we could also use this to dynamically switch a device into
coherent mode. For example, if it is marked as dma-coherent in device
tree but doesn't end up as dev->dma_coherent when the driver probes, we
could try and force coherency via the device's configuration registers.

I don't know yet if that's really a good idea, though. For correctness
it would be enough if we can detect at runtime whether a device is DMA
coherent or not via the IOMMU.

Thierry

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