packet_mmap: How to kernel address from user-space address

I'm trying to accelerate snort with a hardware Pattern Matching Engine. snort gets packets using libpcap which uses packet mmap. snort gets the mmap-ed address, it does all header parsing, now I want to offload the content detection to hardware. PME driver currently provides and ioctl interface to pass the packet. Driver does copy_from_user to get packet data to kernel and try to match it against pre-loaded contents. I'm trying to avoid this copy. What is the best way to get the kernel address from user address in this case?