[PATCH 12/13] kvm/powerpc: Accelerate H_PUT_TCE by implementing it in real mode

[Benjamin Herrenschmidt]

On Tue, 2011-05-17 at 11:31 +0200, Alexander Graf wrote:
> On 17.05.2011, at 11:11, Benjamin Herrenschmidt wrote:
> 
> > On Tue, 2011-05-17 at 10:01 +0200, Alexander Graf wrote:
> >> 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?
> > 
> > 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.
> > 
> > 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.
> > 
> > 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.

Right :-)

> 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?

Oh, bcs qemu mmaps the table :-)

Cheers,
Ben.