riscv: Use PUD/P4D/PGD pages for the linear mapping

riscv: Use PUD/P4D/PGD pages for the linear mapping

During the early page table creation, we used to set the mapping for
PAGE_OFFSET to the kernel load address: but the kernel load address is
always offseted by PMD_SIZE which makes it impossible to use PUD/P4D/PGD
pages as this physical address is not aligned on PUD/P4D/PGD size (whereas
PAGE_OFFSET is).

But actually we don't have to establish this mapping (ie set va_pa_offset)
that early in the boot process because:

- first, setup_vm installs a temporary kernel mapping and among other
  things, discovers the system memory,
- then, setup_vm_final creates the final kernel mapping and takes
  advantage of the discovered system memory to create the linear
  mapping.

During the first phase, we don't know the start of the system memory and
then until the second phase is finished, we can't use the linear mapping at
all and phys_to_virt/virt_to_phys translations must not be used because it
would result in a different translation from the 'real' one once the final
mapping is installed.

So here we simply delay the initialization of va_pa_offset to after the
system memory discovery. But to make sure noone uses the linear mapping
before, we add some guard in the DEBUG_VIRTUAL config.

Finally we can use PUD/P4D/PGD hugepages when possible, which will result
in a better TLB utilization.

Note that:
- this does not apply to rv32 as the kernel mapping lies in the linear
  mapping.
- we rely on the firmware to protect itself using PMP.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Acked-by: Rob Herring <robh@kernel.org> # DT bits
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20230324155421.271544-4-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>