Introduce pt_walk(), which does software page table walking to resolve the
following issues:
1. vmap_to_mfn() uses virt_to_maddr(), which is designed to work with VA
   from either the direct map region or Xen's linkage region (XEN_VIRT_START),
   thereby an assertion will occur if it is used with other regions, in
   particular for the VMAP region. The solution is usage of pt_walk() for
   vmap_to_mfn().
2. pt_mapping_level() returns incorrect page table level in the case when
   mfn==INVALID_MFN when, for example, VA was mapped to PA using 4k mapping,
   but during destroying/modification pt_mapping_level() could return incorrect
   page table level as when mfn==INVALID_MFN then only VA is taking into account
   for page table level calculation and so if VA is page table level 1 aligned
   then page_mapping_level() will return level 1 ( instead of level 0 as VA was
   mapped to PA using 4k mapping so there is incostinency here ).
   The solution is to set level=CONFIG_PAGING_TABLE to tell pt_update() algo
   that it should use pt_walk() to find proper page table entry instead of
   using for searching of page table entry based on precalculated by
   pt_mapping_level() `level` and `order` values.

---
Changes in v5:
 - Change 'patch for 4.20' to 'patch for 4.21'.
 - Update the cover letter message.
 - Other changes please look inside specific patch.
---
Changes in v4:
 - please look at a specific patch.
---
Changes in v2-v3:
 - update the commit message.
 - other changes look in specific patch.
---

Oleksii Kurochko (3):
  xen/riscv: implement software page table walking
  xen/riscv: update defintion of vmap_to_mfn()
  xen/riscv: update mfn calculation in pt_mapping_level()

 xen/arch/riscv/include/asm/mm.h   |   2 +-
 xen/arch/riscv/include/asm/page.h |  11 ++
 xen/arch/riscv/pt.c               | 176 +++++++++++++++++++++++-------
 3 files changed, 150 insertions(+), 39 deletions(-)

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2.48.1