After this, RISC-V fully supports the kmap interface.
Due to the current design limitations of having only a single L2 table
per process, the kernel kmap area cannot be mapped via any user page
directory, as they do not contain the page tables to address that range.
So a "kernel address environment" is added, which can do the mapping. The
mapping is reflected to every process as only the root page directory (L1)
is copied to users, which means every change to L2 / L3 tables will be
seen by every user.
Mapping a physical page to a kernel virtual page is very simple and does
not need the kernel vma list, just get the kernel addressable virtual
address for the page.
is_kmap_vaddr is added and used to test that a given (v)addr is actually
inside the kernel map area. This gives a speed optimization for kmm_unmap,
as it is no longer necessary to take the mm_map_lock to check if such a
mapping exists; obviously if the address is not within the kmap area, it
won't be in the list either.
User pages are mapped from the currently active address environment. If
the process is running on a borrowed address environment, then the
mapping should be created from there.
This happens during (new) process creation only.
This adds functionality to map pages dynamically into kernel virtual
memory. This allows implementing I/O remap for example, which is a useful
(future) feature.
Now, the first target is to support mapping user pages for the kernel.
Why? There are some userspace structures that might be needed when the
userspace process is not running. Semaphores are one such example. Signals
and the WDT timeout both need access to the user semaphore to work
properly. Even though for this only obtaining the kernel addressable
page pool virtual address is needed, for completeness a procedure is
provided to map several pages.
