Re: [PATCH v8] dma-mapping: introduce dma_get_iommu_domain()

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Oct 9, 2017 at 3:37 AM, Robin Murphy <robin.murphy@arm.com> wrote:
> Hi Dan,
>
> On 08/10/17 04:45, Dan Williams wrote:
>> Add a dma-mapping api helper to retrieve the generic iommu_domain for a device.
>> The motivation for this interface is making RDMA transfers to DAX mappings
>> safe. If the DAX file's block map changes we need to be to reliably stop
>> accesses to blocks that have been freed or re-assigned to a new file.
>
> ...which is also going to require some way to force the IOMMU drivers
> (on x86 at least) to do a fully-synchronous unmap, instead of just
> throwing the IOVA onto a flush queue to invalidate the TLBs at some
> point in the future.

Isn't that the difference between iommu_unmap() and
iommu_unmap_fast()? As far as I can tell amd-iommu and intel-iommu
both flush iotlbs on iommu_unmap() and don't support fast unmaps.

> Assuming of course that there's an IOMMU both
> present and performing DMA translation in the first place.

That's why I want to call through the dma api to see if the iommu is
being used to satisfy dma mappings.

>> With the
>> iommu_domain and a callback from the DAX filesystem the kernel can safely
>> revoke access to a DMA device. The process that performed the RDMA memory
>> registration is also notified of this revocation event, but the kernel can not
>> otherwise be in the position of waiting for userspace to quiesce the device.
>
> OK, but why reinvent iommu_get_domain_for_dev()?

How do I know if the iommu returned from that routine is the one being
used for dma mapping operations for the device? Specifically, how
would I discover that the result of dma_map_sg() can be passed as an
IOVA range to iommu_unmap()?

>> Since PMEM+DAX is currently only enabled for x86, we only update the x86
>> iommu drivers.
>
> Note in particular that those two drivers happen to be the *only* place
> this approach could work - everyone else is going to have to fall back
> to the generic IOMMU API function anyway.

I want to make this functionality generic, but I'm not familiar with
the iommu sub-system. How are dma mapping operations routed to the
iommu driver in those other imlementations?