Is there an explict split between userspace and kernel in physical memory on Linux x86-64?

Question

That is, given a physical address, can I tell whether this address is from userspace or not?

As far as I know, in virtual address space, the kernel will use the upp

Answer 1

No.

Almost any physical page frame can be mapped to a userspace virtual address or a kernel virtual address, or even both at the same time.