Re: [PATCH resend v3] riscv: Allow to downgrade paging mode from the command line

[Anup Patel]

On Mon, Jan 23, 2023 at 4:21 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
>
> Add 2 early command line parameters that allow to downgrade satp mode
> (using the same naming as x86):
> - "no5lvl": use a 4-level page table (down from sv57 to sv48)
> - "no4lvl": use a 3-level page table (down from sv57/sv48 to sv39)
>
> Note that going through the device tree to get the kernel command line
> works with ACPI too since the efi stub creates a device tree anyway with
> the command line.
>
> Also, as those params are treated very early in the boot process and we
> use standard device tree functions that may be kasan instrumented, we
> only enable them for !KASAN configurations.
>
> Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxx>
> Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>

Looks good to me.

Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>

Regards,
Anup

> ---
>
> v3:
> - Massage commit log to make no4lvl clearer, as asked by Conor
> - Add a note to kernel-parameters.txt regarding the impossibility to use
>   those parameters when KASAN is enabled, as suggested by Conor
> - Add RB from Björn
>
> v2:
> - Honor CMDLINE_EXTEND and CMDLINE_FORCE as noticed by Björn
>
>  .../admin-guide/kernel-parameters.txt         |  9 ++-
>  arch/riscv/mm/init.c                          | 72 +++++++++++++++++--
>  2 files changed, 74 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 6cfa6e3996cf..fd647412ec91 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3578,8 +3578,15 @@
>                         emulation library even if a 387 maths coprocessor
>                         is present.
>
> -       no5lvl          [X86-64] Disable 5-level paging mode. Forces
> +       no4lvl          [RISCV] Disable 4-level and 5-level paging modes. Forces
> +                       kernel to use 3-level paging instead.
> +                       Note: On RISC-V, this can't be used when KASAN is
> +                       enabled.
> +
> +       no5lvl          [X86-64,RISCV] Disable 5-level paging mode. Forces
>                         kernel to use 4-level paging instead.
> +                       Note: On RISC-V, this can't be used when KASAN is
> +                       enabled.
>
>         nofsgsbase      [X86] Disables FSGSBASE instructions.
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 478d6763a01a..10f99fa74368 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -746,17 +746,77 @@ static void __init disable_pgtable_l4(void)
>         satp_mode = SATP_MODE_39;
>  }
>
> +#ifndef CONFIG_KASAN
> +static __init bool match_noXlvl(const char *cmdline)
> +{
> +       if (strstr(cmdline, "no5lvl")) {
> +               disable_pgtable_l5();
> +       } else if (strstr(cmdline, "no4lvl")) {
> +               disable_pgtable_l5();
> +               disable_pgtable_l4();
> +               return true;
> +       }
> +
> +       return false;
> +}
> +
> +static int __init print_no4lvl(char *p)
> +{
> +       pr_info("Disabled 4-level and 5-level paging");
> +       return 0;
> +}
> +early_param("no4lvl", print_no4lvl);
> +
> +static int __init print_no5lvl(char *p)
> +{
> +       pr_info("Disabled 5-level paging");
> +       return 0;
> +}
> +early_param("no5lvl", print_no5lvl);
> +#endif
> +
>  /*
>   * There is a simple way to determine if 4-level is supported by the
>   * underlying hardware: establish 1:1 mapping in 4-level page table mode
>   * then read SATP to see if the configuration was taken into account
>   * meaning sv48 is supported.
>   */
> -static __init void set_satp_mode(void)
> +static __init void set_satp_mode(uintptr_t dtb_pa)
>  {
>         u64 identity_satp, hw_satp;
>         uintptr_t set_satp_mode_pmd = ((unsigned long)set_satp_mode) & PMD_MASK;
> -       bool check_l4 = false;
> +
> +#ifndef CONFIG_KASAN
> +       /*
> +        * The below fdt functions are kasan instrumented, since at this point
> +        * there is no mapping for the kasan shadow memory, this can't be used
> +        * when kasan is enabled.
> +        */
> +       int chosen_node;
> +       unsigned int fdt_cmdline_size = 0;
> +
> +       if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
> +               chosen_node = fdt_path_offset((void *)dtb_pa, "/chosen");
> +               if (chosen_node >= 0) {
> +                       const char *fdt_cmdline;
> +
> +                       fdt_cmdline = fdt_getprop((void *)dtb_pa, chosen_node,
> +                                                 "bootargs", NULL);
> +                       if (fdt_cmdline) {
> +                               if (match_noXlvl(fdt_cmdline))
> +                                       return;
> +                               fdt_cmdline_size = strlen(fdt_cmdline);
> +                       }
> +               }
> +       }
> +
> +       if (IS_ENABLED(CONFIG_CMDLINE_EXTEND) ||
> +           IS_ENABLED(CONFIG_CMDLINE_FORCE) ||
> +           fdt_cmdline_size == 0 /* CONFIG_CMDLINE_FALLBACK */) {
> +               if (match_noXlvl(CONFIG_CMDLINE))
> +                       return;
> +       }
> +#endif
>
>         create_p4d_mapping(early_p4d,
>                         set_satp_mode_pmd, (uintptr_t)early_pud,
> @@ -775,7 +835,8 @@ static __init void set_satp_mode(void)
>  retry:
>         create_pgd_mapping(early_pg_dir,
>                            set_satp_mode_pmd,
> -                          check_l4 ? (uintptr_t)early_pud : (uintptr_t)early_p4d,
> +                          pgtable_l5_enabled ?
> +                               (uintptr_t)early_p4d : (uintptr_t)early_pud,
>                            PGDIR_SIZE, PAGE_TABLE);
>
>         identity_satp = PFN_DOWN((uintptr_t)&early_pg_dir) | satp_mode;
> @@ -786,9 +847,8 @@ static __init void set_satp_mode(void)
>         local_flush_tlb_all();
>
>         if (hw_satp != identity_satp) {
> -               if (!check_l4) {
> +               if (pgtable_l5_enabled) {
>                         disable_pgtable_l5();
> -                       check_l4 = true;
>                         memset(early_pg_dir, 0, PAGE_SIZE);
>                         goto retry;
>                 }
> @@ -979,7 +1039,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>  #endif
>
>  #if defined(CONFIG_64BIT) && !defined(CONFIG_XIP_KERNEL)
> -       set_satp_mode();
> +       set_satp_mode(dtb_pa);
>  #endif
>
>         kernel_map.va_pa_offset = PAGE_OFFSET - kernel_map.phys_addr;
> --
> 2.37.2
>