Re: [PATCH v4 03/12] iommu: Add attach/detach_dev_pasid domain ops

Jean-Philippe Brucker
2022-04-28T14:54:07Z

On Thu, Apr 21, 2022 at 01:21:12PM +0800, Lu Baolu wrote:
> Attaching an IOMMU domain to a PASID of a device is a generic operation
> for modern IOMMU drivers which support PASID-granular DMA address
> translation. Currently visible usage scenarios include (but not limited):
>
> - SVA (Shared Virtual Address)
> - kernel DMA with PASID
> - hardware-assist mediated device
>
> This adds a pair of common domain ops for this purpose and adds helpers
> to attach/detach a domain to/from a {device, PASID}. Some buses, like
> PCI, route packets without considering the PASID value. Thus a DMA target
> address with PASID might be treated as P2P if the address falls into the
> MMIO BAR of other devices in the group. To make things simple, these
> interfaces only apply to devices belonging to the singleton groups, and
> the singleton is immutable in fabric i.e. not affected by hotplug.
>
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
[...]
> +/*
> + * Use standard PCI bus topology, isolation features, and DMA
> + * alias quirks to check the immutable singleton attribute. If
> + * the device came from DT, assume it is static and then
> + * singleton can know from the device count in the group.
> + */
> +static bool device_group_immutable_singleton(struct device *dev)
> +{
> + struct iommu_group *group = iommu_group_get(dev);
> + int count;
> +
> + if (!group)
> + return false;
> +
> + mutex_lock(&group->mutex);
> + count = iommu_group_device_count(group);
> + mutex_unlock(&group->mutex);
> + iommu_group_put(group);
> +
> + if (count != 1)
> + return false;
> +
> + if (dev_is_pci(dev)) {
> + struct pci_dev *pdev = to_pci_dev(dev);
> +
> + /*
> + * The device could be considered to be fully isolated if
> + * all devices on the path from the device to the host-PCI
> + * bridge are protected from peer-to-peer DMA by ACS.
> + */
> + if (!pci_acs_path_enabled(pdev, NULL, REQ_ACS_FLAGS))
> + return false;
> +
> + /* Filter out devices which has any alias device. */
> + if (pci_for_each_dma_alias(pdev, has_pci_alias, pdev))
> + return false;

Aren't aliases already added to the group by pci_device_group()? If so
it's part of the count check above

> +
> + return true;
> + }
> +
> + /*
> + * If the device came from DT, assume it is static and then
> + * singleton can know from the device count in the group.
> + */
> + return is_of_node(dev_fwnode(dev));

I don't think DT is relevant here because a platform device enumerated
through ACPI will also have its own group. It should be safe to stick to
what the IOMMU drivers declare with their device_group() callback. Except
for PCI those groups should be immutable so we can return true here.

Thanks,
Jean