On 17.05.2011, at 11:11, Benjamin Herrenschmidt wrote:

On Tue, 2011-05-17 at 10:01 +0200, Alexander Graf wrote:

quoted

I'm not sure I fully understand how this is supposed to work. If the
tables are kept inside the kernel, how does userspace get to know
where to DMA to?

=20
The guest gets a dma range from the device-tree which is the range of
device-side dma addresses it can use that correspond to the table.
=20
The guest kernel uses the normal linux iommu space allocator to =

allocate

space in that region and uses H_PUT_TCE to populate the corresponding
table entries.
=20
This is the same interface that is used for "real" iommu's with PCI
devices btw.

I'm still slightly puzzled here :). IIUC the main point of an IOMMU is =
for the kernel to change where device accesses actually go to. So device =
DMAs address A, goes through the IOMMU, in reality accesses address B.

Now, how do we tell the devices implemented in qemu that they're =
supposed to DMA to address B instead of A if the mapping table is kept =
in-kernel?


Alex