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

[Andrew Morton]

(cc trimmed a bit)

On Thu, 13 Nov 2014 11:30:11 +0900 (JST) HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com> wrote:

> 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.

Folks, could we please get a bit of reviewing, acking or nacking for
this one?


From: HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com>
Subject: kdump, vmcoreinfo: report actual value of phys_base

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
VMCOREINFO
OSRELEASE=3.10.0-121.el7.x86_64
PAGESIZE=4096
...

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.

Signed-off-by: HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com>
Cc: Eric W. Biederman <ebiederm at xmission.com>
Cc: Vivek Goyal <vgoyal at redhat.com>
Cc: Atsushi Kumagai <kumagai-atsushi at mxc.nes.nec.co.jp>
Cc: Dave Anderson <anderson at redhat.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
---

 arch/x86/kernel/machine_kexec_64.c |    1 +
 include/linux/kexec.h              |    2 ++
 2 files changed, 3 insertions(+)

diff -puN arch/x86/kernel/machine_kexec_64.c~kdump-vmcoreinfo-report-actual-value-of-phys_base arch/x86/kernel/machine_kexec_64.c
--- a/arch/x86/kernel/machine_kexec_64.c~kdump-vmcoreinfo-report-actual-value-of-phys_base
+++ a/arch/x86/kernel/machine_kexec_64.c
@@ -334,6 +334,7 @@ void arch_crash_save_vmcoreinfo(void)
 #endif
 	vmcoreinfo_append_str("KERNELOFFSET=%lx\n",
 			      (unsigned long)&_text - __START_KERNEL);
+	VMCOREINFO_PHYS_BASE(phys_base);
 }
 
 /* arch-dependent functionality related to kexec file-based syscall */
diff -puN include/linux/kexec.h~kdump-vmcoreinfo-report-actual-value-of-phys_base include/linux/kexec.h
--- a/include/linux/kexec.h~kdump-vmcoreinfo-report-actual-value-of-phys_base
+++ a/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;
_