The de-virtualized Machines (de-VM), That is an experimental FPGA synthesized implementation of a Virtual Machine, needs to work as a co-processor or an accelerator to the main system.
For example, within an Android smart-phone, a synthesized (as Hard IP core) Dalvik Machine will appear as a co-processor. The Java applications would run on it, but for I/O and other OS related services it will transfer control to the main application processor like ARM Cortex-A8 processor running the kernel code.
The main and the co-processor should be as tightly coupled as possible for maximizing performance, however, if there is a design constraint then the co-processor can be loosely coupled.
Tightly Coupled
The best performance can be achieved when the main processor and the co-processor are part of the same system-on-chip, or in case of Intel FPGA strategy can be on separate sockets connected via Front Side Bus (FSB).
The options in the order of performance are:
Hybrid Cores: The main and co-processor are connected through the cache-crossbar switch.
Wishbone: The co-processor connects through Open Cores Wishbone protocol
FSB (Front Side Bus): Intel and Xilinx support FPGA interfacing with main processor via FSB, see the Intel PDF presentation. This option can also be employed in cases where the main processor is FPGA synthesized and exposes its interfaces through FSB. For example, see the ACM paper on Intel Atom core synthesized to a Xilinx Virtex-5 LX-330 FPGA.
Loosely Coupled
In this case the synthesized machine and the main application processor are in separate packaging.
PCI-express:
SDIO :
High Speed USB 2.0: