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    Subject[RFC PATCH 09/12] riscv: Allow user to downgrade to sv39 when hw supports sv48
    Date
    This is made possible by using the mmu-type property of the cpu node of
    the device tree.

    By default, the kernel will boot with 4-level page table if the hw supports
    it but it can be interesting for the user to select 3-level page table as
    it is less memory consuming and faster since it requires less memory
    accesses in case of a TLB miss.

    Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
    Reviewed-by: Anup Patel <anup@brainfault.org>
    ---
    arch/riscv/mm/init.c | 26 ++++++++++++++++++++++++--
    1 file changed, 24 insertions(+), 2 deletions(-)

    diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
    index cb23a30d9af3..f9a99cb1870b 100644
    --- a/arch/riscv/mm/init.c
    +++ b/arch/riscv/mm/init.c
    @@ -550,10 +550,32 @@ void disable_pgtable_l4(void)
    * then read SATP to see if the configuration was taken into account
    * meaning sv48 is supported.
    */
    -asmlinkage __init void set_satp_mode(uintptr_t load_pa)
    +asmlinkage __init void set_satp_mode(uintptr_t load_pa, uintptr_t dtb_pa)
    {
    u64 identity_satp, hw_satp;
    + int cpus_node;

    + /* 1/ Check if the user asked for sv39 explicitly in the device tree */
    + cpus_node = fdt_path_offset((void *)dtb_pa, "/cpus");
    + if (cpus_node >= 0) {
    + int node;
    +
    + fdt_for_each_subnode(node, (void *)dtb_pa, cpus_node) {
    + const char *mmu_type = fdt_getprop((void *)dtb_pa, node,
    + "mmu-type", NULL);
    + if (!mmu_type)
    + continue;
    +
    + if (!strcmp(mmu_type, "riscv,sv39")) {
    + disable_pgtable_l4();
    + return;
    + }
    +
    + break;
    + }
    + }
    +
    + /* 2/ Determine if the HW supports sv48: if not, fallback to sv39 */
    create_pgd_mapping(early_pg_dir, load_pa, (uintptr_t)early_pud,
    PGDIR_SIZE, PAGE_TABLE);
    create_pud_mapping(early_pud, load_pa, (uintptr_t)early_pmd,
    @@ -611,7 +633,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
    #endif

    #if defined(CONFIG_64BIT) && !defined(CONFIG_MAXPHYSMEM_2GB)
    - set_satp_mode(load_pa);
    + set_satp_mode(load_pa, dtb_pa);
    #endif

    kernel_virt_addr = KERNEL_VIRT_ADDR;
    --
    2.20.1
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