Re: [PATCH v8 04/11] bus: platform,amba,fsl-mc,PCI: Add device DMA ownership management

[Robin Murphy]

On 2022-03-08 05:44, Lu Baolu wrote:
> The devices on platform/amba/fsl-mc/PCI buses could be bound to drivers
> with the device DMA managed by kernel drivers or user-space applications.
> Unfortunately, multiple devices may be placed in the same IOMMU group
> because they cannot be isolated from each other. The DMA on these devices
> must either be entirely under kernel control or userspace control, never
> a mixture. Otherwise the driver integrity is not guaranteed because they
> could access each other through the peer-to-peer accesses which by-pass
> the IOMMU protection.
>
> This checks and sets the default DMA mode during driver binding, and
> cleanups during driver unbinding. In the default mode, the device DMA is
> managed by the device driver which handles DMA operations through the
> kernel DMA APIs (see Documentation/core-api/dma-api.rst).
>
> For cases where the devices are assigned for userspace control through the
> userspace driver framework(i.e. VFIO), the drivers(for example, vfio_pci/
> vfio_platfrom etc.) may set a new flag (driver_managed_dma) to skip this
> default setting in the assumption that the drivers know what they are
> doing with the device DMA.
>
> Calling iommu_device_use_default_domain() before {of,acpi}_dma_configure
> is currently a problem. As things stand, the IOMMU driver ignored the
> initial iommu_probe_device() call when the device was added, since at
> that point it had no fwspec yet. In this situation,
> {of,acpi}_iommu_configure() are retriggering iommu_probe_device() after
> the IOMMU driver has seen the firmware data via .of_xlate to learn that
> it actually responsible for the given device. As the result, before
> that gets fixed, iommu_use_default_domain() goes at the end, and calls
> arch_teardown_dma_ops() if it fails.
>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Cc: Stuart Yoder <stuyoder@xxxxxxxxx>
> Cc: Laurentiu Tudor <laurentiu.tudor@xxxxxxx>
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>