RISC-V: Setup initial page tables in two stages
Currently, the setup_vm() does initial page table setup in one-shot very early before enabling MMU. Due to this, the setup_vm() has to map all possible kernel virtual addresses since it does not know size and location of RAM. This means we have kernel mappings for non-existent RAM and any buggy driver (or kernel) code doing out-of-bound access to RAM will not fault and cause underterministic behaviour. Further, the setup_vm() creates PMD mappings (i.e. 2M mappings) for RV64 systems. This means for PAGE_OFFSET=0xffffffe000000000 (i.e. MAXPHYSMEM_128GB=y), the setup_vm() will require 129 pages (i.e. 516 KB) of memory for initial page tables which is never freed. The memory required for initial page tables will further increase if we chose a lower value of PAGE_OFFSET (e.g. 0xffffff0000000000) This patch implements two-staged initial page table setup, as follows: 1. Early (i.e. setup_vm()): This stage maps kernel image and DTB in a early page table (i.e. early_pg_dir). The early_pg_dir will be used only by boot HART so it can be freed as-part of init memory free-up. 2. Final (i.e. setup_vm_final()): This stage maps all possible RAM banks in the final page table (i.e. swapper_pg_dir). The boot HART will start using swapper_pg_dir at the end of setup_vm_final(). All non-boot HARTs directly use the swapper_pg_dir created by boot HART. We have following advantages with this new approach: 1. Kernel mappings for non-existent RAM don't exists anymore. 2. Memory consumed by initial page tables is now indpendent of the chosen PAGE_OFFSET. 3. Memory consumed by initial page tables on RV64 system is 2 pages (i.e. 8 KB) which has significantly reduced and these pages will be freed as-part of the init memory free-up. The patch also provides a foundation for implementing strict kernel mappings where we protect kernel text and rodata using PTE permissions. Suggested-by:Mike Rapoport <rppt@linux.ibm.com> Signed-off-by:
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