Where are mapped device memory to, in virtual addressing, when using Intel I/OAT?

Question

When I use Intel I/OAT for DMA zero-copy/zero-cycles(without CPU) transfer through async_memcpy, then where are mapped device memory to, in virtual addressing: to the kernel-buffer(kernel space) or to the user-buffer(user space)?

And does it make any sense to use I/OAT in modern x86_64 CPUs (when CPU-core can fast access to the RAM without north-bridge of chipset)?

http://www.intel.com/content/www/us/en/wireless-network/accel-technology.html

Answer 1

Given that the memory is physical memory, it can be any memory that the kernel can address, including both kernel buffers and user-space buffers. It does however have to be "pinned" or "locked", so that the memory doesn't get taken away (e.g. someone doing free on the memory should not release the memory back to the OS for reassignment to another process, because you could get very interesting effects if that is the case). This is of course the same rules that apply to various other DMA accesses.

I doubt very much this helps in copying data structures for your average user-mode application. On the other hand, I don't believe Intel would put these sort of features into the processor unless they thought it was beneficial in some way. The way I understand it is that it's helpful for copying the network receive buffer into user-mode application that is receiving the data, with less CPU involvement. It doesn't necessarily speed up the actual memory transfer much (if at all), but it offloads the CPU from the to do other things.

I'm pretty sure I saw something not so long ago about this technology [or something very similar] also going into the latest models of processors, so I expect there is some advantage to it.