[PATCH v2 5/6] powerpc/pseries/iommu: Make use of DDW even if it does not map the partition

[Leonardo Bras]

On Wed, 2020-06-24 at 03:24 -0300, Leonardo Bras wrote:
> As of today, if a DDW is created and can't map the whole partition, it's
> removed and the default DMA window "ibm,dma-window" is used instead.
> 
> Usually this DDW is bigger than the default DMA window, so it would be
> better to make use of it instead.
> 
> Signed-off-by: Leonardo Bras <leobras.c at gmail.com>
> ---

I tested this change with a 256GB DDW which did not map the whole
partition, with a MT27700 Family [ConnectX-4 Virtual Function].

I noticed the performance improvement is about the same as using DDW
with IOMMU bypass.

64 thread write throughput: +203.0%
64 thread read throughput: +17.5%
1 thread write throughput: +20.5%
1 thread read throughput: +3.43%
Averag
e write latency: -23.0%
Average read latency:  -2.26%