Introduce sv48 support

This patchset implements sv48 support at runtime. The kernel will try to         
boot with 4-level page table and will fallback to 3-level if the HW does not     
support it.                                                                      
                                                                                 
The biggest advantage is that we only have one kernel for 64bit, which           
is way easier to maintain.                                                       
                                                                                 
Folding the 4th level into a 3-level page table has almost no cost at            
runtime. But as mentioned Palmer, the relocatable code generated is less         
performant.                                                                      
                                                                                 
At the moment, there is no way to build a 3-level page table non-relocatable     
64bit kernel. We agreed that distributions will use this runtime configuration   
anyway, but Palmer proposed to introduce a new Kconfig, which I will do later    
as sv48 support was asked for 5.8.                                               
                                                                                 
Finally, the user can now ask for sv39 explicitly by using the device-tree       
which will reduce memory footprint and reduce the number of memory accesses      
in case of TLB miss.

Changes in v2:
  * Move variable declarations to pgtable.h in patch 5/7 as suggested by Anup
  * Restore mmu-type properties in patch 6 as suggested by Anup
  * Fix unused variable in patch 5 that was used in patch 6
  * Fix SPARSEMEM build (patch 2 was modified so I dropped the Reviewed-by)
  * Applied various Reviewed-by

Alexandre Ghiti (8):
  riscv: Get rid of compile time logic with MAX_EARLY_MAPPING_SIZE
  riscv: Allow to dynamically define VA_BITS
  riscv: Simplify MAXPHYSMEM config
  riscv: Prepare ptdump for vm layout dynamic addresses
  riscv: Implement sv48 support
  riscv: Allow user to downgrade to sv39 when hw supports sv48
  riscv: Use pgtable_l4_enabled to output mmu type in cpuinfo
  riscv: Explicit comment about user virtual address space size

 arch/riscv/Kconfig                  |  34 ++---
 arch/riscv/include/asm/csr.h        |   3 +-
 arch/riscv/include/asm/fixmap.h     |   1 +
 arch/riscv/include/asm/page.h       |  15 +++
 arch/riscv/include/asm/pgalloc.h    |  36 ++++++
 arch/riscv/include/asm/pgtable-64.h |  97 +++++++++++++-
 arch/riscv/include/asm/pgtable.h    |  31 ++++-
 arch/riscv/include/asm/sparsemem.h  |   6 +-
 arch/riscv/kernel/cpu.c             |  23 ++--
 arch/riscv/kernel/head.S            |   3 +-
 arch/riscv/mm/context.c             |   2 +-
 arch/riscv/mm/init.c                | 194 ++++++++++++++++++++++++----
 arch/riscv/mm/ptdump.c              |  49 +++++--
 13 files changed, 412 insertions(+), 82 deletions(-)

-- 
2.20.1