[PATCH v2] kdump, vmcoreinfo: report actual value of phys_base

HATAYAMA Daisuke d.hatayama at jp.fujitsu.com
Wed Nov 12 18:30:11 PST 2014

Currently, VMCOREINFO note information reports the virtual address of
phys_base that is assigned to symbol phys_base. But this doesn't make
sense because to refer to phys_base, it's necessary to get the value
of phys_base itself we are now about to refer to.

Userland tools related to kdump such as makedumpfile and crash utility
so far have made some efforts to calculate phys_base on crash dump
formats generated by mechanisms running outside Linux kernel, such as
virtual machine hypervisor such as qemu dump, which ordinary users use
via virsh dump, or ones implemented on vendor specific firmware.

That is, find a kernel data whose virtual and physical addresses are
available via its note information and calculate phys_base from
it. However, such data structure is not the one prepared for phys_base
purpose. There's no guarantee that other crash dump mechanisms include
such information that can be used to calculate phys_base similarly.

To get VMCOREINFO in vmcore, it's easy to use strings and grep
commands like this; VMCOREINFO consists of simple string:

$ strings vmcore-3.10.0-121.el7.x86_64 | grep -E ".*VMCOREINFO.*" -A 100

This is also useful to get value of phys_base in kdump 2nd kernel
contained in vmcore using the above-mentioned external crash dump
mechanism; kdump 2nd kernel is an inherently relocated kernel.

This commit doesn't remove VMCOREINFO_SYMBOL(phys_base) line because
makedumpfile refers to it and if removing it, old versions
makedumpfile doesn't work well.

- Correct patch description: this work is primarily for mechanisms
  running outside (kdump 1st) Linux kernel.

Signed-off-by: HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com>
 arch/x86/kernel/machine_kexec_64.c | 1 +
 include/linux/kexec.h              | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index 4859810..47cc835 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -334,6 +334,7 @@ void arch_crash_save_vmcoreinfo(void)
 			      (unsigned long)&_text - __START_KERNEL);
 /* arch-dependent functionality related to kexec file-based syscall */
diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index 9d957b7..bee3c5b 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -258,6 +258,8 @@ unsigned long paddr_vmcoreinfo_note(void);
 	vmcoreinfo_append_str("NUMBER(%s)=%ld\n", #name, (long)name)
 #define VMCOREINFO_CONFIG(name) \
 	vmcoreinfo_append_str("CONFIG_%s=y\n", #name)
+#define VMCOREINFO_PHYS_BASE(value) \
+	vmcoreinfo_append_str("PHYS_BASE=%lx\n", (unsigned long)value)
 extern struct kimage *kexec_image;
 extern struct kimage *kexec_crash_image;

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