Dynamic DMA Window (DDW) supports TCEs that are backed by 2MB page
size. In most configurations, DDW is big enough to pre-map all of LPAR
memory for IO. Pre-mapping of memory for DMA results in improvements in
IO performance.

Persistent memory, vPMEM, can be assigned to an LPAR as well. vPMEM is
not contiguous with LPAR memory and usually is assigned at high memory
addresses.  This makes is not possible to pre-map both vPMEM and LPAR
memory in the same DDW.

For a dedicated adapter this limitation is not an issue. Dedicated
adapters can have both Default DMA window, which is backed by 4K page
size and a DDW backed by 2MB page size TCEs. In this scenario, LPAR
memory is pre-mapped in the DDW.  Any DMA going to the vPMEM is routed
via dynamically allocated TCEs in the default window.

The issue arises with SR-IOV adapters. There is only one DMA window -
either Default or DDW. If an LPAR has vPMEM assigned, memory is not
pre-mapped in the DDW since TCEs needs to be allocated for vPMEM as well.
In this case, DDW is created and TCEs are dynamically allocated for both
vPMEM and LPAR memory.

Today, DDW is only used in single mode - direct mapped TCEs or
dynamically mapped TCEs. This enhancement breaks a single DDW in 2
regions -

	1. First region to pre-map LPAR memory
	2. Second region to dynamically allocate TCEs for IO to vPMEM

The DDW is split only if it is big enough to pre-map complete LPAR
memory and still have some space left to dynamically map vPMEM. Maximum
size possible DDW is created as permitted by the Hypervisor.
Signed-off-by: Gaurav Batra <gbatra@linux.ibm.com>
Reviewed-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240514014608.35537-1-gbatra@linux.ibm.com