linux-kernel - Re: [PATCH 2/2] riscv: Use PUD/PGDIR entries for linear mapping when possible

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Date:   Thu, 18 Jun 2020 17:47:30 -0700
From:   Atish Patra <atishp@...shpatra.org>
To:     Alexandre Ghiti <alex@...ti.fr>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <Atish.Patra@....com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] riscv: Use PUD/PGDIR entries for linear mapping when possible

On Wed, Jun 3, 2020 at 8:38 AM Alexandre Ghiti <alex@...ti.fr> wrote:
>
> Improve best_map_size so that PUD or PGDIR entries are used for linear
> mapping when possible as it allows better TLB utilization.
>
> Signed-off-by: Alexandre Ghiti <alex@...ti.fr>
> ---
>  arch/riscv/mm/init.c | 45 +++++++++++++++++++++++++++++++++-----------
>  1 file changed, 34 insertions(+), 11 deletions(-)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 9a5c97e091c1..d275f9f834cf 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -424,13 +424,29 @@ static void __init create_pgd_mapping(pgd_t *pgdp,
>         create_pgd_next_mapping(nextp, va, pa, sz, prot);
>  }
>
> -static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
> +static bool is_map_size_ok(uintptr_t map_size, phys_addr_t base,
> +                          uintptr_t base_virt, phys_addr_t size)
>  {
> -       /* Upgrade to PMD_SIZE mappings whenever possible */
> -       if ((base & (PMD_SIZE - 1)) || (size & (PMD_SIZE - 1)))
> -               return PAGE_SIZE;
> +       return !((base & (map_size - 1)) || (base_virt & (map_size - 1)) ||
> +                       (size < map_size));
> +}
> +
> +static uintptr_t __init best_map_size(phys_addr_t base, uintptr_t base_virt,
> +                                     phys_addr_t size)
> +{
> +#ifndef __PAGETABLE_PMD_FOLDED
> +       if (is_map_size_ok(PGDIR_SIZE, base, base_virt, size))
> +               return PGDIR_SIZE;
> +
> +       if (pgtable_l4_enabled)
> +               if (is_map_size_ok(PUD_SIZE, base, base_virt, size))
> +                       return PUD_SIZE;
> +#endif
> +
> +       if (is_map_size_ok(PMD_SIZE, base, base_virt, size))
> +               return PMD_SIZE;
>
> -       return PMD_SIZE;
> +       return PAGE_SIZE;
>  }
>
>  /*
> @@ -576,7 +592,7 @@ void create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
>  asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>  {
>         uintptr_t va, end_va;
> -       uintptr_t map_size = best_map_size(load_pa, MAX_EARLY_MAPPING_SIZE);
> +       uintptr_t map_size;
>
>         load_pa = (uintptr_t)(&_start);
>         load_sz = (uintptr_t)(&_end) - load_pa;
> @@ -587,6 +603,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>
>         kernel_virt_addr = KERNEL_VIRT_ADDR;
>
> +       map_size = best_map_size(load_pa, PAGE_OFFSET, MAX_EARLY_MAPPING_SIZE);
>         va_pa_offset = PAGE_OFFSET - load_pa;
>         va_kernel_pa_offset = kernel_virt_addr - load_pa;
>         pfn_base = PFN_DOWN(load_pa);
> @@ -700,6 +717,8 @@ static void __init setup_vm_final(void)
>
>         /* Map all memory banks */
>         for_each_memblock(memory, reg) {
> +               uintptr_t remaining_size;
> +
>                 start = reg->base;
>                 end = start + reg->size;
>
> @@ -707,15 +726,19 @@ static void __init setup_vm_final(void)
>                         break;
>                 if (memblock_is_nomap(reg))
>                         continue;
> -               if (start <= __pa(PAGE_OFFSET) &&
> -                   __pa(PAGE_OFFSET) < end)
> -                       start = __pa(PAGE_OFFSET);
>
> -               map_size = best_map_size(start, end - start);
> -               for (pa = start; pa < end; pa += map_size) {
> +               pa = start;
> +               remaining_size = reg->size;
> +
> +               while (remaining_size) {
>                         va = (uintptr_t)__va(pa);
> +                       map_size = best_map_size(pa, va, remaining_size);
> +
>                         create_pgd_mapping(swapper_pg_dir, va, pa,
>                                            map_size, PAGE_KERNEL);
> +
> +                       pa += map_size;
> +                       remaining_size -= map_size;
>                 }
>         }
>

This may not work in the RV32 with 2G memory  and if the map_size is
determined to be a page size
for the last memblock. Both pa & remaining_size will overflow and the
loop will try to map memory from zero again.

> --
> 2.20.1
>
>


-- 
Regards,
Atish