On 09/07/2016 06:29 AM, AKASHI Takahiro wrote:
On crash dump kernel, all the information about primary kernel's system
memory (core image) is available in elf core header.
The primary kernel will set aside this header with reserve_elfcorehdr()
at boot time and inform crash dump kernel of its location via a new
device-tree property, "linux,elfcorehdr".

Please note that all other architectures use traditional "elfcorehdr="
kernel parameter for this purpose.

Then crash dump kernel will access the primary kernel's memory with
copy_oldmem_page(), which reads one page by ioremap'ing it since it does
not reside in linear mapping on crash dump kernel.

We also need our own elfcorehdr_read() here since the header is placed
within crash dump kernel's usable memory.

Signed-off-by: AKASHI Takahiro <takahiro.aka...@linaro.org>
---
 arch/arm64/Kconfig             | 11 +++++++
 arch/arm64/kernel/Makefile     |  1 +
 arch/arm64/kernel/crash_dump.c | 71 ++++++++++++++++++++++++++++++++++++++++++
 arch/arm64/mm/init.c           | 54 ++++++++++++++++++++++++++++++++
 4 files changed, 137 insertions(+)
 create mode 100644 arch/arm64/kernel/crash_dump.c

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index bc3f00f..9c15c66 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -683,6 +683,17 @@ config KEXEC
          but it is independent of the system firmware.   And like a reboot
          you can start any kernel with it, not just Linux.

+config CRASH_DUMP
+       bool "Build kdump crash kernel"
+       help
+         Generate crash dump after being started by kexec. This should
+         be normally only set in special crash dump kernels which are
+         loaded in the main kernel with kexec-tools into a specially
+         reserved region and then later executed after a crash by
+         kdump/kexec.
+
+         For more details see Documentation/kdump/kdump.txt
+
 config XEN_DOM0
        def_bool y
        depends on XEN
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 14f7b65..f1cbfc8 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -48,6 +48,7 @@ arm64-obj-$(CONFIG_RANDOMIZE_BASE)    += kaslr.o
 arm64-obj-$(CONFIG_HIBERNATION)                += hibernate.o hibernate-asm.o
 arm64-obj-$(CONFIG_KEXEC)              += machine_kexec.o relocate_kernel.o    
\
                                           cpu-reset.o
+arm64-obj-$(CONFIG_CRASH_DUMP)         += crash_dump.o

 obj-y                                  += $(arm64-obj-y) vdso/ probes/
 obj-m                                  += $(arm64-obj-m)
diff --git a/arch/arm64/kernel/crash_dump.c b/arch/arm64/kernel/crash_dump.c
new file mode 100644
index 0000000..bc5b932
--- /dev/null
+++ b/arch/arm64/kernel/crash_dump.c
@@ -0,0 +1,71 @@
+/*
+ * Routines for doing kexec-based kdump
+ *
+ * Copyright (C) 2014 Linaro Limited
+ * Author: AKASHI Takahiro <takahiro.aka...@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/crash_dump.h>
+#include <linux/errno.h>
+#include <linux/io.h>
+#include <linux/memblock.h>
+#include <linux/uaccess.h>
+#include <asm/memory.h>
+
+/**
+ * copy_oldmem_page() - copy one page from old kernel memory
+ * @pfn: page frame number to be copied
+ * @buf: buffer where the copied page is placed
+ * @csize: number of bytes to copy
+ * @offset: offset in bytes into the page
+ * @userbuf: if set, @buf is in a user address space
+ *
+ * This function copies one page from old kernel memory into buffer pointed by
+ * @buf. If @buf is in userspace, set @userbuf to %1. Returns number of bytes
+ * copied or negative error in case of failure.
+ */
+ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
+                        size_t csize, unsigned long offset,
+                        int userbuf)
+{
+       void *vaddr;
+
+       if (!csize)
+               return 0;
+
+       vaddr = memremap(__pfn_to_phys(pfn), PAGE_SIZE, MEMREMAP_WB);
+       if (!vaddr)
+               return -ENOMEM;
+
+       if (userbuf) {
+               if (copy_to_user(buf, vaddr + offset, csize)) {
+                       memunmap(vaddr);
+                       return -EFAULT;
+               }
+       } else {
+               memcpy(buf, vaddr + offset, csize);
+       }
+
+       memunmap(vaddr);
+
+       return csize;
+}
+
+/**
+ * elfcorehdr_read - read from ELF core header
+ * @buf: buffer where the data is placed
+ * @csize: number of bytes to read
+ * @ppos: address in the memory
+ *
+ * This function reads @count bytes from elf core header which exists
+ * on crash dump kernel's memory.
+ */
+ssize_t elfcorehdr_read(char *buf, size_t count, u64 *ppos)
+{
+       memcpy(buf, phys_to_virt((phys_addr_t)*ppos), count);
+       return count;
+}
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index dd273ec..e4d9c38 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -36,6 +36,7 @@
 #include <linux/efi.h>
 #include <linux/swiotlb.h>
 #include <linux/kexec.h>
+#include <linux/crash_dump.h>

 #include <asm/boot.h>
 #include <asm/fixmap.h>
@@ -186,6 +187,57 @@ static void __init reserve_crashkernel(void)
 }
 #endif /* CONFIG_KEXEC_CORE */

+#ifdef CONFIG_CRASH_DUMP
+static int __init early_init_dt_scan_elfcorehdr(unsigned long node,
+               const char *uname, int depth, void *data)
+{
+       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 elf core header given in "elfcorehdr=" kernel
+ * command line parameter. 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);
+

Do I get that right that we can pass crashkernel address/size through the kernel boot parameter, but elfcorehdr can only be provided via device tree?
Why? If there is a reason for doing so, we should fix the documentation.

Regards,
Matthias

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