How to keep PCI-e endpoints and RCs in distinct IOMMU groups?

Hey there,

We're experiencing an issue with IOMMU groups and PCI-e devices. The system in question has a WLAN DMA master behind a PCI-e root complex which is, in turn, behind an IOMMU. There are no there devices behind the RC. This is on an ARM platform using the arm-smmu and pci-msm drivers (pci-msm is in the MSM vendor tree, sorry...).

What we're observing is that the WLAN endpoint device is being added to the same IOMMU group as the root complex device itself. I don't think they should be in the same group though, since they each have different BDFs, which, in our system, are translated to different SMMU Stream IDs, so their traffic is split onto separate SMMU context banks. Since their traffic is isolated from one other I don't think they need to be in the same IOMMU group (as I understand IOMMU groups).

The result is that when the WLAN driver tries to attach to their IOMMU it errors out due to the following check in iommu_attach_device:

        if (iommu_group_device_count(group) != 1)
                goto out_unlock;

I've come up with a few hacky workarounds:

- Forcing PCI-e ACS to be "enabled" unconditionally (even though our platform doesn't actually support it).

- Call iommu_attach_group instead of iommu_attach_device in the arm64 DMA IOMMU mapping layer (yuck).

- Don't use the pci_device_group helper at all from the arm-smmu driver. Just allocate a new group for all PCI-e devices.

It seems like the proper solution would be to somehow make these devices end up in separate IOMMU groups using the existing pci_device_group helper, since that might be doing useful stuff for other configurations (like detecting the DMA aliasing quirks).

Looking at pci_device_group, though, I'm not sure how we could tell that these two devices are supposed to get separated. I know very little about PCI-e so maybe I'm just missing something simple. The match happens in the following loop where we walk up the PCI-e topology:

        /*
         * Continue upstream from the point of minimum IOMMU granularity
         * due to aliases to the point where devices are protected from
         * peer-to-peer DMA by PCI ACS.  Again, if we find an existing
         * group, use it.
         */
        for (bus = pdev->bus; !pci_is_root_bus(bus); bus = bus->parent) {
                if (!bus->self)
                        continue;

                if (pci_acs_path_enabled(bus->self, NULL, REQ_ACS_FLAGS))
                        break;

                pdev = bus->self;

                group = iommu_group_get(&pdev->dev);
                if (group)
                        return group;
        }

Why do we do that? If the devices have different BDFs can't we safely say that they're protected from peer-to-peer DMA (assuming no DMA aliasing quirks)? Even as I write that out it seems wrong though since the RC can probably do whatever it wants...

Maybe the IOMMU framework can't actually know whether the devices should be kept in separate groups and we just need to do something custom in the arm-smmu driver?

Quoted 8 lines

Why do we do that? If the devices have different BDFs can't we safely say that they're protected from peer-to-peer DMA (assuming no DMA aliasing quirks)? Even as I write that out it seems wrong though since the RC can probably do whatever it wants...

Maybe the IOMMU framework can't actually know whether the devices should be kept in separate groups and we just need to do something custom in the arm-smmu driver?

You're only considering the visibility of devices to the IOMMU, not the isolation between devices. Without ACS peer-to-peer can be re-routed between devices before the IOMMU even knows about it. That's why the root port is included in the group. I'm confused why your driver is using the IOMMU API instead of the much more common DMA API anyway though. Thanks,

Alex

Hey there,

We're experiencing an issue with IOMMU groups and PCI-e devices. The system in question has a WLAN DMA master behind a PCI-e root complex which is, in turn, behind an IOMMU. There are no there devices behind the RC. This is on an ARM platform using the arm-smmu and pci-msm drivers (pci-msm is in the MSM vendor tree, sorry...).

What we're observing is that the WLAN endpoint device is being added to the same IOMMU group as the root complex device itself. I don't think they should be in the same group though, since they each have different BDFs, which, in our system, are translated to different SMMU Stream IDs, so their traffic is split onto separate SMMU context banks. Since their traffic is isolated from one other I don't think they need to be in the same IOMMU group (as I understand IOMMU groups).

The result is that when the WLAN driver tries to attach to their IOMMU it errors out due to the following check in iommu_attach_device:

         if (iommu_group_device_count(group) != 1)
                 goto out_unlock;

I've come up with a few hacky workarounds:

- Forcing PCI-e ACS to be "enabled" unconditionally (even though our platform doesn't actually support it).

- Call iommu_attach_group instead of iommu_attach_device in the arm64 DMA IOMMU mapping layer (yuck).

- Don't use the pci_device_group helper at all from the arm-smmu driver. Just allocate a new group for all PCI-e devices.

It seems like the proper solution would be to somehow make these devices end up in separate IOMMU groups using the existing pci_device_group helper, since that might be doing useful stuff for other configurations (like detecting the DMA aliasing quirks).

Looking at pci_device_group, though, I'm not sure how we could tell that these two devices are supposed to get separated. I know very little about PCI-e so maybe I'm just missing something simple. The match happens in the following loop where we walk up the PCI-e topology:

         /*
          * Continue upstream from the point of minimum IOMMU granularity
          * due to aliases to the point where devices are protected from
          * peer-to-peer DMA by PCI ACS.  Again, if we find an existing
          * group, use it.
          */
         for (bus = pdev->bus; !pci_is_root_bus(bus); bus = bus->parent) {
                 if (!bus->self)
                         continue;

                 if (pci_acs_path_enabled(bus->self, NULL, REQ_ACS_FLAGS))
                         break;

                 pdev = bus->self;

                 group = iommu_group_get(&pdev->dev);
                 if (group)
                         return group;
         }

Why do we do that? If the devices have different BDFs can't we safely say that they're protected from peer-to-peer DMA (assuming no DMA aliasing quirks)? Even as I write that out it seems wrong though since the RC can probably do whatever it wants...

Maybe the IOMMU framework can't actually know whether the devices should be kept in separate groups and we just need to do something custom in the arm-smmu driver?

Sorry for the novel! Thanks for any pointers.

-Mitch

Hey Mitch,

Quoted 19 lines · Mitchel Humpherys

Hey there,

We're experiencing an issue with IOMMU groups and PCI-e devices. The system in question has a WLAN DMA master behind a PCI-e root complex which is, in turn, behind an IOMMU. There are no there devices behind the RC. This is on an ARM platform using the arm-smmu and pci-msm drivers (pci-msm is in the MSM vendor tree, sorry...).

What we're observing is that the WLAN endpoint device is being added to the same IOMMU group as the root complex device itself. I don't think they should be in the same group though, since they each have different BDFs, which, in our system, are translated to different SMMU Stream IDs, so their traffic is split onto separate SMMU context banks. Since their traffic is isolated from one other I don't think they need to be in the same IOMMU group (as I understand IOMMU groups).

The result is that when the WLAN driver tries to attach to their IOMMU it errors out due to the following check in iommu_attach_device:

Quoted 7 lines

         if (iommu_group_device_count(group) != 1)
                 goto out_unlock;

I've come up with a few hacky workarounds:

- Forcing PCI-e ACS to be "enabled" unconditionally (even though our platform doesn't actually support it).

If the _only_ use of the IOMMU is to allow 32-bit devices to get at physically higher RAM without DAC addressing, then perhaps. If system integrity matters, though, you're opening up the big hole that Alex mentions. I'm reminded of Rob Clark's awesome Fire TV hack for some of the dangers of letting DMA-capable devices play together without careful supervision...

Quoted 2 lines

- Call iommu_attach_group instead of iommu_attach_device in the arm64 DMA IOMMU mapping layer (yuck).

That's not yuck, that would be correct, except for the arm64 DMA mapping code relying on default domains from the IOMMU core and not calling iommu_attach anything :/

If you've not picked 921b1f52c942 into the MSM kernel, please do so and fix the fallout in whatever other modifications you have. That dodgy workaround was only necessary for the brief window between the DMA mapping code and the IOMMU core group rework both landing in 4.4, and then hung around unused for far too long, frankly.

Quoted 3 lines

- Don't use the pci_device_group helper at all from the arm-smmu driver. Just allocate a new group for all PCI-e devices.

See point #1.

Quoted 30 lines

It seems like the proper solution would be to somehow make these devices end up in separate IOMMU groups using the existing pci_device_group helper, since that might be doing useful stuff for other configurations (like detecting the DMA aliasing quirks).

Looking at pci_device_group, though, I'm not sure how we could tell that these two devices are supposed to get separated. I know very little about PCI-e so maybe I'm just missing something simple. The match happens in the following loop where we walk up the PCI-e topology:

         /*
          * Continue upstream from the point of minimum IOMMU granularity
          * due to aliases to the point where devices are protected from
          * peer-to-peer DMA by PCI ACS.  Again, if we find an existing
          * group, use it.
          */
         for (bus = pdev->bus; !pci_is_root_bus(bus); bus = bus->parent) {
                 if (!bus->self)
                         continue;

                 if (pci_acs_path_enabled(bus->self, NULL, REQ_ACS_FLAGS))
                         break;

                 pdev = bus->self;

                 group = iommu_group_get(&pdev->dev);
                 if (group)
                         return group;
         }

Quoted 4 lines

Why do we do that? If the devices have different BDFs can't we safely say that they're protected from peer-to-peer DMA (assuming no DMA aliasing quirks)? Even as I write that out it seems wrong though since the RC can probably do whatever it wants...

Quite ;)

Quoted 3 lines

Maybe the IOMMU framework can't actually know whether the devices should be kept in separate groups and we just need to do something custom in the arm-smmu driver?

From my perspective, things are to the contrary - the IOMMU core assumes devices should be in separate groups unless it _does_ know otherwise, and the ARM SMMU driver is severely lacking in the cases where devices do need grouping in ways the core can't discover - I guess you've not had the pleasure of watching multiple platform devices on the same stream ID blowing up.

I am of course addressing this in my SMMU generic bindings patches (v2 coming real soon now) - having said which I'm now doing a double-take because until that series there are no IOMMU DMA ops for PCI devices and no real DMA mapping support for the SMMU, so... how... this would appear to be a problem entirely belonging to out-of-tree code :P