Re: [PATCH V1] riscv: mm: Support > 1GB kernel image size when creating early page table

[Alexandre Ghiti]

Hi Yang,

On Tue, Apr 30, 2024 at 11:24 AM yang.zhang <gaoshanliukou@xxxxxxx> wrote:
>
> From: "yang.zhang" <yang.zhang@xxxxxxxxxxxx>
>
> By default, when creating early page table, only one PMD page table, but
> if kernel image size exceeds 1GB, it need two PMD page table, otherwise,
> it would BUG_ON in create_kernel_page_table.
>
> In addition, if trap earlier, trap vector doesn't yet set properly, current
> value maybe set by previous firmwire, typically it's the _start of kernel,
> it's confused and difficult to debuge, so set it earlier.

Totally agree with that, I'm used to debugging this part of the code
and I know stvec points to __start and not handle_exception at this
point...But that deserves a fix indeed, it's confusing. I don't see
this fix in the patch below though?

> ---
> I'm not sure whether hugesize kernel is reasonable, if not, ignore this patch.

Unless you have a good use case for this, I don't think that's
necessary, we never encountered such situations so I'd rather not
complicate this code even more.

Thanks,

Alex

> This issue can be reproduced simpily by changing '_end' in vmlinux.lds.S
> such as _end = . + 0x40000000；
>
> Signed-off-by: yang.zhang <yang.zhang@xxxxxxxxxxxx>
> ---
>  arch/riscv/mm/init.c | 41 +++++++++++++++++++++++++++++++++++------
>  1 file changed, 35 insertions(+), 6 deletions(-)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index fe8e159394d8..094b39f920d3 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -386,11 +386,13 @@ static void __init create_pte_mapping(pte_t *ptep,
>  static pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
>  static pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
>  static pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
> +static pmd_t early_pmd2[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
>
>  #ifdef CONFIG_XIP_KERNEL
>  #define trampoline_pmd ((pmd_t *)XIP_FIXUP(trampoline_pmd))
>  #define fixmap_pmd     ((pmd_t *)XIP_FIXUP(fixmap_pmd))
>  #define early_pmd      ((pmd_t *)XIP_FIXUP(early_pmd))
> +#define early_pmd2      ((pmd_t *)XIP_FIXUP(early_pmd2))
>  #endif /* CONFIG_XIP_KERNEL */
>
>  static p4d_t trampoline_p4d[PTRS_PER_P4D] __page_aligned_bss;
> @@ -432,9 +434,14 @@ static pmd_t *__init get_pmd_virt_late(phys_addr_t pa)
>
>  static phys_addr_t __init alloc_pmd_early(uintptr_t va)
>  {
> -       BUG_ON((va - kernel_map.virt_addr) >> PUD_SHIFT);
> +       uintptr_t end_pud_idx = pud_index(kernel_map.virt_addr + kernel_map.size - 1);
> +       uintptr_t current_pud_idx = pud_index(va);
>
> -       return (uintptr_t)early_pmd;
> +       BUG_ON(current_pud_idx > end_pud_idx);
> +       if (current_pud_idx == end_pud_idx)
> +               return (uintptr_t)early_pmd2;
> +       else
> +               return (uintptr_t)early_pmd;
>  }
>
>  static phys_addr_t __init alloc_pmd_fixmap(uintptr_t va)
> @@ -769,6 +776,18 @@ static void __init set_mmap_rnd_bits_max(void)
>         mmap_rnd_bits_max = MMAP_VA_BITS - PAGE_SHIFT - 3;
>  }
>
> +static pmd_t *__init select_pmd_early(uintptr_t pa)
> +{
> +       uintptr_t end_pud_idx = pud_index(kernel_map.phys_addr + kernel_map.size - 1);
> +       uintptr_t current_pud_idx = pud_index(pa);
> +
> +       BUG_ON(current_pud_idx > end_pud_idx);
> +       if (current_pud_idx == end_pud_idx)
> +               return early_pmd2;
> +       else
> +               return early_pmd;
> +}
> +
>  /*
>   * 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
> @@ -780,6 +799,7 @@ 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;
>         u64 satp_mode_cmdline = __pi_set_satp_mode_from_cmdline(dtb_pa);
> +       pmd_t *target_pmd, *target_pmd2;
>
>         if (satp_mode_cmdline == SATP_MODE_57) {
>                 disable_pgtable_l5();
> @@ -789,17 +809,24 @@ static __init void set_satp_mode(uintptr_t dtb_pa)
>                 return;
>         }
>
> +       target_pmd = select_pmd_early(set_satp_mode_pmd);
> +       target_pmd2 = select_pmd_early(set_satp_mode_pmd + PMD_SIZE);
>         create_p4d_mapping(early_p4d,
>                         set_satp_mode_pmd, (uintptr_t)early_pud,
>                         P4D_SIZE, PAGE_TABLE);
>         create_pud_mapping(early_pud,
> -                          set_satp_mode_pmd, (uintptr_t)early_pmd,
> +                          set_satp_mode_pmd, (uintptr_t)target_pmd,
> +                          PUD_SIZE, PAGE_TABLE);
> +       /* Handle the case where set_satp_mode straddles 2 PUDs */
> +       if (target_pmd2 != target_pmd)
> +               create_pud_mapping(early_pud,
> +                          set_satp_mode_pmd + PMD_SIZE, (uintptr_t)target_pmd2,
>                            PUD_SIZE, PAGE_TABLE);
>         /* Handle the case where set_satp_mode straddles 2 PMDs */
> -       create_pmd_mapping(early_pmd,
> +       create_pmd_mapping(target_pmd,
>                            set_satp_mode_pmd, set_satp_mode_pmd,
>                            PMD_SIZE, PAGE_KERNEL_EXEC);
> -       create_pmd_mapping(early_pmd,
> +       create_pmd_mapping(target_pmd2,
>                            set_satp_mode_pmd + PMD_SIZE,
>                            set_satp_mode_pmd + PMD_SIZE,
>                            PMD_SIZE, PAGE_KERNEL_EXEC);
> @@ -829,7 +856,9 @@ static __init void set_satp_mode(uintptr_t dtb_pa)
>         memset(early_pg_dir, 0, PAGE_SIZE);
>         memset(early_p4d, 0, PAGE_SIZE);
>         memset(early_pud, 0, PAGE_SIZE);
> -       memset(early_pmd, 0, PAGE_SIZE);
> +       memset(target_pmd, 0, PAGE_SIZE);
> +       if (target_pmd2 != target_pmd)
> +               memset(target_pmd2, 0, PAGE_SIZE);
>  }
>  #endif
>
> --
> 2.25.1
>