Re: [PATCH 6/8] riscv: Allow user to downgrade to sv39 when hw supports sv48

[Anup Patel]

On Sun, May 24, 2020 at 2:46 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> 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@xxxxxxxx>
> ---
>  arch/riscv/mm/init.c | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index bad8da099ff6..1776eeb53d61 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -509,11 +509,32 @@ 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,
> @@ -561,7 +582,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>         load_sz = (uintptr_t)(&_end) - load_pa;
>
>  #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
>

Looks good to me.

Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>

Regards,
Anup