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Subject: Re: [PATCH 2/2] riscv: Use PUD/PGDIR entries for linear mapping when
 possible
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>,
        Alistair Francis <Alistair.Francis@wdc.com>
References: <20200603153608.30056-1-alex@ghiti.fr>
 <20200603153608.30056-3-alex@ghiti.fr>
 <CAOnJCU+JSuOGbOmZW-vqb-A_qR7CJc=qG16FbgOLWSm1vhJH1A@mail.gmail.com>
 <23529a84-44a0-3c45-f16d-5a7ee528610d@ghiti.fr>
 <CAOnJCU+s5JuNdPg_R-Cg2+WnMjR51DD0ekbRr84EFCig6=YyZA@mail.gmail.com>
 <f1a5ec6e-540b-497f-a9ad-f2d1e7adfc65@ghiti.fr>
 <2588a00a-b042-4902-1602-7cb8d587ac2b@ghiti.fr>
 <CAOnJCU+JOdoJfVCAKOHK52m47UwR_NzpJoGXQywD+Mx-6JRw5w@mail.gmail.com>
From:   Alex Ghiti <alex@ghiti.fr>
Message-ID: <567064d8-0f23-1629-c40a-606a89cf4b97@ghiti.fr>
Date:   Mon, 29 Jun 2020 10:42:12 -0400
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Hi Atish,

Le 6/22/20 à 3:11 PM, Atish Patra a écrit :
> On Sun, Jun 21, 2020 at 2:39 AM Alex Ghiti <alex@ghiti.fr> wrote:
>>
>> 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.
>>
> Yes. But the memblock size is not guaranteed to be PAGE_SIZE aligned.
> The memblock size is updated in memblock_cap_size
> 
>      /* adjust *@size so that (@base + *@size) doesn't overflow, return
> new size */
>     static inline phys_addr_t memblock_cap_size(phys_addr_t base,
> phys_addr_t *size)
>     {
>              return *size = min(*size, PHYS_ADDR_MAX - base);
>     }
> 

Yes you're right, I will fix that in a v2.

Thanks,

Alex

> You will not see this issue right away even if you allocate 2GB of
> memory while running 32 bit linux in qemu
> because the kernel removes anything beyond 0xc0400000 for 32 bit in
> bootmem setup.
> 
> │[    0.000000][    T0] memblock_remove: [0xc0400000-0xfffffffe]
> setup_bootmem+0x90/0x216
> 
> This also restricts the kernel to use only 1GB of memory even if
> maximum physical memory supported is 2GB.
> 
>> 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
>>>>>>>
>>>>>>>
>>>>
>>>>
>>>
> 
> 
> 
> --
> Regards,
> Atish
>