Re: [PATCH v3] ARM: Parse kdump DT properties

[Rob Herring]

On Wed, Mar 17, 2021 at 5:31 AM Geert Uytterhoeven
<geert+renesas@xxxxxxxxx> wrote:
>
> Parse the following DT properties in the crash dump kernel, to provide a
> modern interface between kexec and the crash dump kernel:
>   - linux,elfcorehdr: ELF core header segment, similar to the
>     "elfcorehdr=" kernel parameter.
>   - linux,usable-memory-range: Usable memory reserved for the crash dump
>     kernel.
>     This 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, and
> the "elfcorehdr=" and "mem=" kernel parameters are still parsed.
>
> Loosely based on the ARM64 version by Akashi Takahiro.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> ---
> 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/linux-arm-kernel/20200902154129.6358-1-geert+renesas@xxxxxxxxx/
>
> 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,
> ---
>  Documentation/devicetree/bindings/chosen.txt  |  4 +-

Note that I've been meaning to remove this file because dtschema has a
schema replacing it.

>  .../arm/boot/compressed/fdt_check_mem_start.c | 48 ++++++++--
>  arch/arm/mm/init.c                            | 90 +++++++++++++++++++
>  3 files changed, 134 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
> index 45e79172a646c537..ba75c58413667760 100644
> --- a/Documentation/devicetree/bindings/chosen.txt
> +++ b/Documentation/devicetree/bindings/chosen.txt
> @@ -79,7 +79,7 @@ a different secondary CPU release mechanism)
>  linux,usable-memory-range
>  -------------------------
>
> -This property (arm64 only) holds a base address and size, describing a
> +This property (arm and arm64 only) holds a base address and size, describing a

I'd rather not have to change this again when RiscV gets around to
copying kdump support.

>  limited region in which memory may be considered available for use by
>  the kernel. Memory outside of this range is not available for use.
>
> @@ -106,7 +106,7 @@ respectively, of the root node.
>  linux,elfcorehdr
>  ----------------
>
> -This property (currently used only on arm64) holds the memory range,
> +This property (currently used only on arm and arm64) holds the memory range,
>  the address and the size, of the elf core header which mainly describes
>  the panicked kernel's memory layout as PT_LOAD segments of elf format.
>  e.g.
> 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.
>          */
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 828a2561b2295813..d1ae15d9b29121e6 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -4,6 +4,7 @@
>   *
>   *  Copyright (C) 1995-2005 Russell King
>   */
> +#include <linux/crash_dump.h>
>  #include <linux/kernel.h>
>  #include <linux/errno.h>
>  #include <linux/swap.h>
> @@ -210,8 +211,95 @@ void check_cpu_icache_size(int cpuid)
>  }
>  #endif
>
> +#ifdef CONFIG_OF_EARLY_FLATTREE
> +static int __init early_init_dt_scan_usablemem(unsigned long node,
> +               const char *uname, int depth, void *data)
> +{
> +       struct memblock_region *usablemem = data;
> +       const __be32 *reg;
> +       int len;
> +
> +       if (depth != 1 || strcmp(uname, "chosen") != 0)
> +               return 0;

We have libfdt now, just get the '/chosen' node rather than using
of_scan_flat_dt().


> +
> +       reg = of_get_flat_dt_prop(node, "linux,usable-memory-range", &len);
> +       if (!reg || (len < (dt_root_addr_cells + dt_root_size_cells)))
> +               return 1;
> +
> +       usablemem->base = dt_mem_next_cell(dt_root_addr_cells, &reg);
> +       usablemem->size = dt_mem_next_cell(dt_root_size_cells, &reg);
> +       return 1;
> +}
> +
> +static void __init fdt_enforce_memory_region(void)
> +{
> +       struct memblock_region reg = {
> +               .size = 0,
> +       };
> +
> +       of_scan_flat_dt(early_init_dt_scan_usablemem, &reg);
> +
> +       if (reg.size)
> +               memblock_cap_memory_range(reg.base, reg.size);

We should be able to do this in the DT core code. It doesn't matter
that these properties are arm* only. Other arches won't find the
properties.

Also, note that there is now a drivers/of/kexec.c (in -next) though
not sure if all this would go there or stay in fdt.c with the rest of
the memory parsing.

> +}
> +
> +#else
> +static inline void fdt_enforce_memory_region(void) { }
> +#endif
> +
> +#if defined(CONFIG_CRASH_DUMP) && defined(CONFIG_OF_EARLY_FLATTREE)
> +static int __init early_init_dt_scan_elfcorehdr(unsigned long node,
> +               const char *uname, int depth, void *data)

Same comments as above.

> +{
> +       const __be32 *reg;
> +       int len;
> +
> +       if (depth != 1 || strcmp(uname, "chosen") != 0)
> +               return 0;
> +
> +       reg = of_get_flat_dt_prop(node, "linux,elfcorehdr", &len);
> +       if (!reg || (len < (dt_root_addr_cells + dt_root_size_cells)))
> +               return 1;
> +
> +       elfcorehdr_addr = dt_mem_next_cell(dt_root_addr_cells, &reg);
> +       elfcorehdr_size = dt_mem_next_cell(dt_root_size_cells, &reg);
> +       return 1;
> +}
> +
> +/*
> + * reserve_elfcorehdr() - reserves memory for elf core header
> + *
> + * This function reserves the memory occupied by an elf core header
> + * described in the device tree. This region contains all the
> + * information about primary kernel's core image and is used by a dump
> + * capture kernel to access the system memory on primary kernel.
> + */
> +static void __init reserve_elfcorehdr(void)
> +{
> +       of_scan_flat_dt(early_init_dt_scan_elfcorehdr, NULL);
> +
> +       if (!elfcorehdr_size)
> +               return;
> +
> +       if (memblock_is_region_reserved(elfcorehdr_addr, elfcorehdr_size)) {
> +               pr_warn("elfcorehdr is overlapped\n");
> +               return;
> +       }
> +
> +       memblock_reserve(elfcorehdr_addr, elfcorehdr_size);
> +
> +       pr_info("Reserving %llu KiB of memory at 0x%llx for elfcorehdr\n",
> +               elfcorehdr_size >> 10, elfcorehdr_addr);
> +}
> +#else
> +static inline void reserve_elfcorehdr(void) { }
> +#endif
> +
>  void __init arm_memblock_init(const struct machine_desc *mdesc)
>  {
> +       /* Handle linux,usable-memory-range property */
> +       fdt_enforce_memory_region();
> +
>         /* Register the kernel text, kernel data and initrd with memblock. */
>         memblock_reserve(__pa(KERNEL_START), KERNEL_END - KERNEL_START);
>
> @@ -225,6 +313,8 @@ void __init arm_memblock_init(const struct machine_desc *mdesc)
>
>         early_init_fdt_scan_reserved_mem();
>
> +       reserve_elfcorehdr();
> +
>         /* reserve memory for DMA contiguous allocations */
>         dma_contiguous_reserve(arm_dma_limit);
>
> --
> 2.25.1
>