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

[Atish Patra <atishp@atishpatra.org>]

On Thu, Jun 18, 2020 at 9:28 PM Alex Ghiti <alex@ghiti.fr> wrote:
>
> Hi Atish,
>
> Le 6/18/20 à 8:47 PM, Atish Patra a écrit :
> > On Wed, Jun 3, 2020 at 8:38 AM Alexandre Ghiti <alex@ghiti.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@ghiti.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.
>
> I'm not sure I understand: if pa starts at 0x8000_0000 and size is 2G,
> then pa will overflow in the last iteration, but remaining_size will
> then be equal to 0 right ?
>
Not unless the remaining_size is at least page size aligned. The last
remaining size would "fff".
It will overflow as well after subtracting the map_size.

> And by the way, I realize that this loop only handles sizes that are
> aligned on map_size.
>

Yeah.

> Thanks,
>
> Alex
>
>
> >
> >> --
> >> 2.20.1
> >>
> >>
> >



-- 
Regards,
Atish