• Anup Patel's avatar
    RISC-V: Setup initial page tables in two stages · 671f9a3e
    Anup Patel authored
    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: default avatarMike Rapoport <rppt@linux.ibm.com>
    Signed-off-by: default avatarAnup Patel <anup.patel@wdc.com>
    [paul.walmsley@sifive.com: updated to apply; fixed a checkpatch warning]
    Signed-off-by: default avatarPaul Walmsley <paul.walmsley@sifive.com>