Simultaneously use OpenCL buffer in host and kernel

Question

After creating the OpenCL buffer, we need to map it on host side, populate the required data and unmap so that kernel can use it. For a read only OpenCL buffer, is it possible to use it on host side as well as kernel side simultaneously?

Answer 1

No, not if you're using map/unmap. The content of the host memory range is invalid after the unmap. Perhaps you could use clEnqueueWriteBuffer instead, and then the host memory that you used as the source will still be host memory you can use on the host side.

Answer 2

Again, not with regular memory. In general, you can share memory between the GPU and CPU concurrently even to communicate. Look into Shared Virtual Memory (AMD and Intel).

Non-standard CPU/GPU communication is pretty rare for the simple fact that one doesn't get to assume what order the ND range executes. An implementation can dispatch them in any order it desires. So if the buffer's contents were to be changing as the kernel dispatches new workgroups, you wouldn't have any control of the dataflow sequence.

Rare exceptions like "persistent kernels" where the kernel continues running (as if processing a stream) do exist, but I know less about this.