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

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 25 May 2016 17:26:15 -0700
Mitchel Humpherys <mitchelh@codeaurora.org> wrote:

> 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?

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