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Subject: Re: [PATCH 2/2] riscv: Use PUD/PGDIR entries for linear mapping when
 possible
From:   Alex Ghiti <alex@ghiti.fr>
To:     Atish Patra <atishp@atishpatra.org>
Cc:     Anup Patel <anup@brainfault.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>,
        Atish Patra <Atish.Patra@wdc.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        linux-riscv <linux-riscv@lists.infradead.org>
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Date:   Sun, 21 Jun 2020 05:39:51 -0400
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Hi Atish,

Le 6/20/20 à 5:04 AM, Alex Ghiti a écrit :
> Hi Atish,
>
> Le 6/19/20 à 2:16 PM, Atish Patra a écrit :
>> 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.


While fixing this issue, I noticed that if the size in the device tree 
is not aligned on PAGE_SIZE, the size is then automatically realigned on 
PAGE_SIZE: see early_init_dt_add_memory_arch where size is and-ed with 
PAGE_MASK to remove the unaligned part.

So the issue does not need to be fixed :)

Thanks anyway,

Alex


>>
>>> And by the way, I realize that this loop only handles sizes that are
>>> aligned on map_size.
>>>
>> Yeah.
>
>
> Thanks for noticing, I send a v2.
>
> Alex
>
>
>>
>>> Thanks,
>>>
>>> Alex
>>>
>>>
>>>>> -- 
>>>>> 2.20.1
>>>>>
>>>>>
>>
>>
>