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References: <20230324155421.271544-1-alexghiti@rivosinc.com> <20230324155421.271544-4-alexghiti@rivosinc.com>
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From:   Anup Patel <anup@brainfault.org>
Date:   Mon, 27 Mar 2023 17:43:32 +0530
Message-ID: <CAAhSdy3j38AZt7BOipiW_b+t-NZg6xVHEAhOfJLU6TQzSSdcXQ@mail.gmail.com>
Subject: Re: [PATCH v9 3/3] riscv: Use PUD/P4D/PGD pages for the linear mapping
To:     Alexandre Ghiti <alexghiti@rivosinc.com>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Andrew Jones <ajones@ventanamicro.com>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        devicetree@vger.kernel.org, Rob Herring <robh@kernel.org>
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On Fri, Mar 24, 2023 at 9:27=E2=80=AFPM Alexandre Ghiti <alexghiti@rivosinc=
.com> wrote:
>
> During the early page table creation, we used to set the mapping for
> PAGE_OFFSET to the kernel load address: but the kernel load address is
> always offseted by PMD_SIZE which makes it impossible to use PUD/P4D/PGD
> pages as this physical address is not aligned on PUD/P4D/PGD size (wherea=
s
> PAGE_OFFSET is).
>
> But actually we don't have to establish this mapping (ie set va_pa_offset=
)
> that early in the boot process because:
>
> - first, setup_vm installs a temporary kernel mapping and among other
>   things, discovers the system memory,
> - then, setup_vm_final creates the final kernel mapping and takes
>   advantage of the discovered system memory to create the linear
>   mapping.
>
> During the first phase, we don't know the start of the system memory and
> then until the second phase is finished, we can't use the linear mapping =
at
> all and phys_to_virt/virt_to_phys translations must not be used because i=
t
> would result in a different translation from the 'real' one once the fina=
l
> mapping is installed.
>
> So here we simply delay the initialization of va_pa_offset to after the
> system memory discovery. But to make sure noone uses the linear mapping
> before, we add some guard in the DEBUG_VIRTUAL config.
>
> Finally we can use PUD/P4D/PGD hugepages when possible, which will result
> in a better TLB utilization.
>
> Note that:
> - this does not apply to rv32 as the kernel mapping lies in the linear
>   mapping.
> - we rely on the firmware to protect itself using PMP.
>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> Acked-by: Rob Herring <robh@kernel.org> # DT bits

Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>

Regards,
Anup

> ---
>  arch/riscv/include/asm/page.h | 16 ++++++++++
>  arch/riscv/mm/init.c          | 58 +++++++++++++++++++++++++++++++----
>  arch/riscv/mm/physaddr.c      | 16 ++++++++++
>  drivers/of/fdt.c              | 11 ++++---
>  4 files changed, 90 insertions(+), 11 deletions(-)
>
> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.=
h
> index 8dc686f549b6..ea1a0e237211 100644
> --- a/arch/riscv/include/asm/page.h
> +++ b/arch/riscv/include/asm/page.h
> @@ -90,6 +90,14 @@ typedef struct page *pgtable_t;
>  #define PTE_FMT "%08lx"
>  #endif
>
> +#ifdef CONFIG_64BIT
> +/*
> + * We override this value as its generic definition uses __pa too early =
in
> + * the boot process (before kernel_map.va_pa_offset is set).
> + */
> +#define MIN_MEMBLOCK_ADDR      0
> +#endif
> +
>  #ifdef CONFIG_MMU
>  #define ARCH_PFN_OFFSET                (PFN_DOWN((unsigned long)phys_ram=
_base))
>  #else
> @@ -121,7 +129,11 @@ extern phys_addr_t phys_ram_base;
>  #define is_linear_mapping(x)   \
>         ((x) >=3D PAGE_OFFSET && (!IS_ENABLED(CONFIG_64BIT) || (x) < PAGE=
_OFFSET + KERN_VIRT_SIZE))
>
> +#ifndef CONFIG_DEBUG_VIRTUAL
>  #define linear_mapping_pa_to_va(x)     ((void *)((unsigned long)(x) + ke=
rnel_map.va_pa_offset))
> +#else
> +void *linear_mapping_pa_to_va(unsigned long x);
> +#endif
>  #define kernel_mapping_pa_to_va(y)     ({                               =
       \
>         unsigned long _y =3D (unsigned long)(y);                         =
         \
>         (IS_ENABLED(CONFIG_XIP_KERNEL) && _y < phys_ram_base) ?          =
       \
> @@ -130,7 +142,11 @@ extern phys_addr_t phys_ram_base;
>         })
>  #define __pa_to_va_nodebug(x)          linear_mapping_pa_to_va(x)
>
> +#ifndef CONFIG_DEBUG_VIRTUAL
>  #define linear_mapping_va_to_pa(x)     ((unsigned long)(x) - kernel_map.=
va_pa_offset)
> +#else
> +phys_addr_t linear_mapping_va_to_pa(unsigned long x);
> +#endif
>  #define kernel_mapping_va_to_pa(y) ({                                   =
       \
>         unsigned long _y =3D (unsigned long)(y);                         =
         \
>         (IS_ENABLED(CONFIG_XIP_KERNEL) && _y < kernel_map.virt_addr + XIP=
_OFFSET) ? \
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 3b37d8606920..f803671d18b2 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -213,6 +213,14 @@ static void __init setup_bootmem(void)
>         phys_ram_end =3D memblock_end_of_DRAM();
>         if (!IS_ENABLED(CONFIG_XIP_KERNEL))
>                 phys_ram_base =3D memblock_start_of_DRAM();
> +
> +       /*
> +        * In 64-bit, any use of __va/__pa before this point is wrong as =
we
> +        * did not know the start of DRAM before.
> +        */
> +       if (IS_ENABLED(CONFIG_64BIT))
> +               kernel_map.va_pa_offset =3D PAGE_OFFSET - phys_ram_base;
> +
>         /*
>          * memblock allocator is not aware of the fact that last 4K bytes=
 of
>          * the addressable memory can not be mapped because of IS_ERR_VAL=
UE
> @@ -667,9 +675,16 @@ void __init create_pgd_mapping(pgd_t *pgdp,
>
>  static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size=
)
>  {
> -       /* Upgrade to PMD_SIZE mappings whenever possible */
> -       base &=3D PMD_SIZE - 1;
> -       if (!base && size >=3D PMD_SIZE)
> +       if (!(base & (PGDIR_SIZE - 1)) && size >=3D PGDIR_SIZE)
> +               return PGDIR_SIZE;
> +
> +       if (!(base & (P4D_SIZE - 1)) && size >=3D P4D_SIZE)
> +               return P4D_SIZE;
> +
> +       if (!(base & (PUD_SIZE - 1)) && size >=3D PUD_SIZE)
> +               return PUD_SIZE;
> +
> +       if (!(base & (PMD_SIZE - 1)) && size >=3D PMD_SIZE)
>                 return PMD_SIZE;
>
>         return PAGE_SIZE;
> @@ -978,11 +993,22 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>         set_satp_mode();
>  #endif
>
> -       kernel_map.va_pa_offset =3D PAGE_OFFSET - kernel_map.phys_addr;
> +       /*
> +        * In 64-bit, we defer the setup of va_pa_offset to setup_bootmem=
,
> +        * where we have the system memory layout: this allows us to alig=
n
> +        * the physical and virtual mappings and then make use of PUD/P4D=
/PGD
> +        * for the linear mapping. This is only possible because the kern=
el
> +        * mapping lies outside the linear mapping.
> +        * In 32-bit however, as the kernel resides in the linear mapping=
,
> +        * setup_vm_final can not change the mapping established here,
> +        * otherwise the same kernel addresses would get mapped to differ=
ent
> +        * physical addresses (if the start of dram is different from the
> +        * kernel physical address start).
> +        */
> +       kernel_map.va_pa_offset =3D IS_ENABLED(CONFIG_64BIT) ?
> +                               0UL : PAGE_OFFSET - kernel_map.phys_addr;
>         kernel_map.va_kernel_pa_offset =3D kernel_map.virt_addr - kernel_=
map.phys_addr;
>
> -       phys_ram_base =3D kernel_map.phys_addr;
> -
>         /*
>          * The default maximal physical memory size is KERN_VIRT_SIZE for=
 32-bit
>          * kernel, whereas for 64-bit kernel, the end of the virtual addr=
ess
> @@ -1106,6 +1132,17 @@ static void __init create_linear_mapping_page_tabl=
e(void)
>         phys_addr_t start, end;
>         u64 i;
>
> +#ifdef CONFIG_STRICT_KERNEL_RWX
> +       phys_addr_t ktext_start =3D __pa_symbol(_start);
> +       phys_addr_t ktext_size =3D __init_data_begin - _start;
> +       phys_addr_t krodata_start =3D __pa_symbol(__start_rodata);
> +       phys_addr_t krodata_size =3D _data - __start_rodata;
> +
> +       /* Isolate kernel text and rodata so they don't get mapped with a=
 PUD */
> +       memblock_mark_nomap(ktext_start,  ktext_size);
> +       memblock_mark_nomap(krodata_start, krodata_size);
> +#endif
> +
>         /* Map all memory banks in the linear mapping */
>         for_each_mem_range(i, &start, &end) {
>                 if (start >=3D end)
> @@ -1118,6 +1155,15 @@ static void __init create_linear_mapping_page_tabl=
e(void)
>
>                 create_linear_mapping_range(start, end);
>         }
> +
> +#ifdef CONFIG_STRICT_KERNEL_RWX
> +       create_linear_mapping_range(ktext_start, ktext_start + ktext_size=
);
> +       create_linear_mapping_range(krodata_start,
> +                                   krodata_start + krodata_size);
> +
> +       memblock_clear_nomap(ktext_start,  ktext_size);
> +       memblock_clear_nomap(krodata_start, krodata_size);
> +#endif
>  }
>
>  static void __init setup_vm_final(void)
> diff --git a/arch/riscv/mm/physaddr.c b/arch/riscv/mm/physaddr.c
> index 9b18bda74154..18706f457da7 100644
> --- a/arch/riscv/mm/physaddr.c
> +++ b/arch/riscv/mm/physaddr.c
> @@ -33,3 +33,19 @@ phys_addr_t __phys_addr_symbol(unsigned long x)
>         return __va_to_pa_nodebug(x);
>  }
>  EXPORT_SYMBOL(__phys_addr_symbol);
> +
> +phys_addr_t linear_mapping_va_to_pa(unsigned long x)
> +{
> +       BUG_ON(!kernel_map.va_pa_offset);
> +
> +       return ((unsigned long)(x) - kernel_map.va_pa_offset);
> +}
> +EXPORT_SYMBOL(linear_mapping_va_to_pa);
> +
> +void *linear_mapping_pa_to_va(unsigned long x)
> +{
> +       BUG_ON(!kernel_map.va_pa_offset);
> +
> +       return ((void *)((unsigned long)(x) + kernel_map.va_pa_offset));
> +}
> +EXPORT_SYMBOL(linear_mapping_pa_to_va);
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index d1a68b6d03b3..d14735a81301 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -887,12 +887,13 @@ const void * __init of_flat_dt_match_machine(const =
void *default_match,
>  static void __early_init_dt_declare_initrd(unsigned long start,
>                                            unsigned long end)
>  {
> -       /* ARM64 would cause a BUG to occur here when CONFIG_DEBUG_VM is
> -        * enabled since __va() is called too early. ARM64 does make use
> -        * of phys_initrd_start/phys_initrd_size so we can skip this
> -        * conversion.
> +       /*
> +        * __va() is not yet available this early on some platforms. In t=
hat
> +        * case, the platform uses phys_initrd_start/phys_initrd_size ins=
tead
> +        * and does the VA conversion itself.
>          */
> -       if (!IS_ENABLED(CONFIG_ARM64)) {
> +       if (!IS_ENABLED(CONFIG_ARM64) &&
> +           !(IS_ENABLED(CONFIG_RISCV) && IS_ENABLED(CONFIG_64BIT))) {
>                 initrd_start =3D (unsigned long)__va(start);
>                 initrd_end =3D (unsigned long)__va(end);
>                 initrd_below_start_ok =3D 1;
> --
> 2.37.2
>