Re: [PATCH v6] ARM: uncompress: Parse "linux,usable-memory-range" DT property

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 15 Sept 2021 at 15:20, Geert Uytterhoeven
<geert+renesas@glider.be> wrote:
>
> Add support for parsing the "linux,usable-memory-range" DT property.
> This property is used to describe the usable memory reserved for the
> crash dump kernel, and thus makes the memory reservation explicit.
> If present, Linux no longer needs to mask the program counter, and rely
> on the "mem=" kernel parameter to obtain the start and size of usable
> memory.
>
> For backwards compatibility, the traditional method to derive the start
> of memory is still used if "linux,usable-memory-range" is absent.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

Acked-by: Ard Biesheuvel <ardb@kernel.org>

> ---
> KernelVersion: v5.15-rc1
> ---
> The corresponding patch for kexec-tools is "[PATCH] arm: kdump: Add DT
> properties to crash dump kernel's DTB", which is still valid:
> https://lore.kernel.org/r/20200902154129.6358-1-geert+renesas@glider.be/
>
> v6:
>   - All dependencies are in v5.15-rc1,
>
> v5:
>   - Remove the addition of "linux,elfcorehdr" and
>     "linux,usable-memory-range" handling to arch/arm/mm/init.c,
>
> v4:
>   - Remove references to architectures in chosen.txt, to avoid having to
>     change this again when more architectures copy kdump support,
>   - Remove the architecture-specific code for parsing
>     "linux,usable-memory-range" and "linux,elfcorehdr", as the FDT core
>     code now takes care of this,
>   - Move chosen.txt change to patch changing the FDT core,
>   - Use IS_ENABLED(CONFIG_CRASH_DUMP) instead of #ifdef,
>
> v3:
>   - Rebase on top of accepted solution for DTB memory information
>     handling, which is part of v5.12-rc1,
>
> v2:
>   - Rebase on top of reworked DTB memory information handling.
> ---
>  .../arm/boot/compressed/fdt_check_mem_start.c | 48 ++++++++++++++++---
>  1 file changed, 42 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm/boot/compressed/fdt_check_mem_start.c b/arch/arm/boot/compressed/fdt_check_mem_start.c
> index 62450d824c3ca180..9291a2661bdfe57f 100644
> --- a/arch/arm/boot/compressed/fdt_check_mem_start.c
> +++ b/arch/arm/boot/compressed/fdt_check_mem_start.c
> @@ -55,16 +55,17 @@ static uint64_t get_val(const fdt32_t *cells, uint32_t ncells)
>   * DTB, and, if out-of-range, replace it by the real start address.
>   * To preserve backwards compatibility (systems reserving a block of memory
>   * at the start of physical memory, kdump, ...), the traditional method is
> - * always used if it yields a valid address.
> + * used if it yields a valid address, unless the "linux,usable-memory-range"
> + * property is present.
>   *
>   * Return value: start address of physical memory to use
>   */
>  uint32_t fdt_check_mem_start(uint32_t mem_start, const void *fdt)
>  {
> -       uint32_t addr_cells, size_cells, base;
> +       uint32_t addr_cells, size_cells, usable_base, base;
>         uint32_t fdt_mem_start = 0xffffffff;
> -       const fdt32_t *reg, *endp;
> -       uint64_t size, end;
> +       const fdt32_t *usable, *reg, *endp;
> +       uint64_t size, usable_end, end;
>         const char *type;
>         int offset, len;
>
> @@ -80,6 +81,27 @@ uint32_t fdt_check_mem_start(uint32_t mem_start, const void *fdt)
>         if (addr_cells > 2 || size_cells > 2)
>                 return mem_start;
>
> +       /*
> +        * Usable memory in case of a crash dump kernel
> +        * This property describes a limitation: memory within this range is
> +        * only valid when also described through another mechanism
> +        */
> +       usable = get_prop(fdt, "/chosen", "linux,usable-memory-range",
> +                         (addr_cells + size_cells) * sizeof(fdt32_t));
> +       if (usable) {
> +               size = get_val(usable + addr_cells, size_cells);
> +               if (!size)
> +                       return mem_start;
> +
> +               if (addr_cells > 1 && fdt32_ld(usable)) {
> +                       /* Outside 32-bit address space */
> +                       return mem_start;
> +               }
> +
> +               usable_base = fdt32_ld(usable + addr_cells - 1);
> +               usable_end = usable_base + size;
> +       }
> +
>         /* Walk all memory nodes and regions */
>         for (offset = fdt_next_node(fdt, -1, NULL); offset >= 0;
>              offset = fdt_next_node(fdt, offset, NULL)) {
> @@ -107,7 +129,20 @@ uint32_t fdt_check_mem_start(uint32_t mem_start, const void *fdt)
>
>                         base = fdt32_ld(reg + addr_cells - 1);
>                         end = base + size;
> -                       if (mem_start >= base && mem_start < end) {
> +                       if (usable) {
> +                               /*
> +                                * Clip to usable range, which takes precedence
> +                                * over mem_start
> +                                */
> +                               if (base < usable_base)
> +                                       base = usable_base;
> +
> +                               if (end > usable_end)
> +                                       end = usable_end;
> +
> +                               if (end <= base)
> +                                       continue;
> +                       } else if (mem_start >= base && mem_start < end) {
>                                 /* Calculated address is valid, use it */
>                                 return mem_start;
>                         }
> @@ -123,7 +158,8 @@ uint32_t fdt_check_mem_start(uint32_t mem_start, const void *fdt)
>         }
>
>         /*
> -        * The calculated address is not usable.
> +        * The calculated address is not usable, or was overridden by the
> +        * "linux,usable-memory-range" property.
>          * Use the lowest usable physical memory address from the DTB instead,
>          * and make sure this is a multiple of 2 MiB for phys/virt patching.
>          */
> --
> 2.25.1
>