Re: [PATCH v4 10/10] ARM: Parse kdump DT properties

[Rob Herring <robh@kernel.org>]

On Wed, Jul 14, 2021 at 02:50:20PM +0200, Geert Uytterhoeven 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@glider.be>
> ---
> 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@glider.be/
> 
> 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 ++++++++++++++++---
>  arch/arm/mm/init.c                            | 30 ++++++++++++
>  2 files changed, 72 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.
>  	 */
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 6162a070a4104a26..dfaee199554dda97 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>
> @@ -221,8 +222,35 @@ void check_cpu_icache_size(int cpuid)
>  }
>  #endif
>  
> +/*
> + * 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)
> +{
> +	if (!IS_ENABLED(CONFIG_CRASH_DUMP) || !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);

Can't this be in common code?

> +
> +	pr_info("Reserving %llu KiB of memory at 0x%llx for elfcorehdr\n",
> +		elfcorehdr_size >> 10, elfcorehdr_addr);
> +}
> +
>  void __init arm_memblock_init(const struct machine_desc *mdesc)
>  {
> +	/* Handle linux,usable-memory-range property */
> +	memblock_cap_memory_range(cap_mem_addr, cap_mem_size);
> +
>  	/* Register the kernel text, kernel data and initrd with memblock. */
>  	memblock_reserve(__pa(KERNEL_START), KERNEL_END - KERNEL_START);
>  
> @@ -236,6 +264,8 @@ 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
> 
>