Add buffer read/write commands to command-buffer extension

[EwanC]

The SYCL-Graph vendor extension in SYCL is layered on top of the cl_khr_command_buffer OpenCL extension. It currently cannot support the following graph nodes on OpenCL backends as there is no clCommandReadBuffer / clCommandWriteBuffer commands defined by cl_khr_command_buffer.

• handler::copy(src, dest) - Where src is an accessor and dest is a pointer. This corresponds to a memory buffer read command.
• handler::copy(src, dest) - Where src is an pointer and dest is an accessor. This corresponds to a memory buffer write command.

This is a feature request to add command append entry-points to cl_khr_command_buffer to support this, which could be optional to support if desired. PoCL vendor extension cl_pocl_command_buffer_host_buffer already exists to support this, so cc @pjaaskel

[pjaaskel]

+1. Notifying @franz .

[bashbaug]

Confirming: this feature would support reading into and writing from arbitrary host pointers, correct? The pointers would not need to be SVM or USM pointers?

I agree this is useful, but it also brings a surprising amount of complexity. For example, when is the host allowed to read from or write to the host pointers? When is the host allowed to free memory? Are there any requirements (e.g. alignment) for the host pointers?

If we relax this behavior then we should also consider relaxing the restriction on clCommandSVMMemcpyKHR (and clCommandSVMFillKHR?) to allow them to use arbitrary host pointers also.

One more question: Are these copy nodes required for SYCL-Graph? If so, this support would be required to support SYCL-Graph, even if it's optional in cl_khr_command_buffer, correct?