DMA between User Space Buffer and Memory Mapped File

Question

memcpy via user space DMA: Is it possible achieve memcpy via user space DMA in linux ? I know there are ways to copy kernel buffers to user space buffer via DMA but would like to extend that for memcpy via DMA in user space ? Any thoughts ?

Answer 1

DMA operates on physical memory and memory-mapped devices, whereas most user space applications operate on virtual memory. This presents two problems:

• Mapping virtual memory to physical memory: you'll have to make sure it's cache flushed, paged in, locked so it can't be paged out again, and it's unlikely to be contiguous - it'll be scattered all over physical memory in small chunks
• Security of physical memory access: this gives user space applications access to the whole of physical memory, great for sniffing passwords and crypto keys

On a modern processor, memcpy() is likely to be pretty efficient in both software and hardware until you're copying 10s of megabytes at a time, after which you need to think carefully about cacheing and virtual memory anyway. In short, I wouldn't bother.