"If you’re performing this IPC as part of a system call from userspace, the memory containing the original
message is unlikely to be mapped in the receiver’s address space anyway, so we have to copy it into the kernel’s
address space, which is mapped in both processes." PG 226

But it is my understanding that whilst in usermode (CPL = 3) the process cannot directly access memory in the higher half addresses (e.g., 0xFFFFFFFF80000000 and above) because those addresses are typically reserved for the kernel and mapped with page table permissions that prevent user-space access.

TODO:

• Discuss
• Find reference in Intel Manual
• PR with proposed changes