Before we describe the various user-level DMA mechanisms,
we will discuss the notion of
shadow addressing, that is common to all user-level DMA solutions,
and has been proposed (under various names) in [3, 8].
The method of shadow addressing
is used to securely translate virtual to physical addresses
and pass them to the
DMA engine from user-level processes.
For each virtual address vaddr that
is mapped in the physical address paddr, there is also
a shadow address shadow(vaddr), which is mapped in the
shadow physical address {shadow(paddr).
The shadow function is simple and known to the DMA engine.
One simple shadow function is to concatenate each address with an
extra shadow bit. When the shadow bit is set, then the address
is a shadow one.
For example, 0x0FFFFFFFF is a regular 33-bit address, while
0x1FFFFFFFF is its shadow address.
An access to a shadow address is always interpreted by the DMA engine
as a special argument passing operation.
For example, suppose that virtual address vaddr is mapped to
physical address paddr, and that the
virtual address shadow(vaddr) is mapped into shadow(paddr).
Normally, a load (store) operation to
virtual address vaddr by a user application
is translated by the TLB (page-table) into a load (store) operation to
physical address paddr and is performed by the appropriate memory
controller.
Similarly, a load (store) operation
to virtual address shadow(vaddr) is translated by the TLB into
a load (store) operation to physical address shadow(paddr).
When, however, this operation reaches the DMA engine it will be treated
as an argument passing operation, and neither a load nor a store
operation will be performed to physical address shadow(paddr).
Thus, when the user application wants to pass to the DMA engine
the physical address paddr, it makes an access to
virtual address shadow(vaddr). Eventually, the access mode
along with the physical address shadow(paddr) reach the DMA
engine. The DMA engine recognizes the shadow address and takes the
physical address paddr by applying function shadow 
to physical address shadow(paddr).