powerpc: Support for relocatable kdump kernel

This adds relocatable kernel support for kdump. With this one can
use the same regular kernel to capture the kdump. A signature (0xfeed1234)
is passed in r6 from panic code to the next kernel through kexec_sequence
and purgatory code. The signature is used to differentiate between
kdump kernel and non-kdump kernels.

The purgatory code compares the signature and sets the __kdump_flag in
head_64.S.  During the boot up, kernel code checks __kdump_flag and if it
is set, the kernel will behave as relocatable kdump kernel. This kernel
will boot at the address where it was loaded by kexec-tools ie. at the
address reserved through crashkernel boot parameter.

CONFIG_CRASH_DUMP depends on CONFIG_RELOCATABLE option to build kdump
kernel as relocatable. So the same kernel can be used as production and
kdump kernel.

This patch incorporates the changes suggested by Paul Mackerras to avoid
GOT use and to avoid two copies of the code.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Mohan Kumar M <mohan@in.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 3 insertions, 7 deletions

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 369d93e7377c..5b1527883fcb 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -323,13 +323,11 @@ config KEXEC
 
 config CRASH_DUMP
         bool "Build a kdump crash kernel"
-        depends on PPC_MULTIPLATFORM && PPC64
+        depends on PPC_MULTIPLATFORM && PPC64 && RELOCATABLE
         help
           Build a kernel suitable for use as a kdump capture kernel.
-          The kernel will be linked at a different address than normal, and
-          so can only be used for Kdump.
-
-          Don't change this unless you know what you are doing.
+          The same kernel binary can be used as production kernel and dump
+          capture kernel.
 
 config PHYP_DUMP
         bool "Hypervisor-assisted dump (EXPERIMENTAL)"
@@ -829,11 +827,9 @@ config PAGE_OFFSET
         default "0xc000000000000000"
 config KERNEL_START
         hex
-        default "0xc000000002000000" if CRASH_DUMP
         default "0xc000000000000000"
 config PHYSICAL_START
         hex
-        default "0x02000000" if CRASH_DUMP
         default "0x00000000"
 endif