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, 9 insertions, 4 deletions

diff --git a/arch/powerpc/kernel/machine_kexec_64.c b/arch/powerpc/kernel/machine_kexec_64.c
index 4bd8b4f5e70d..e6efec788c4d 100644
--- a/arch/powerpc/kernel/machine_kexec_64.c
+++ b/arch/powerpc/kernel/machine_kexec_64.c
@@ -255,11 +255,14 @@ static union thread_union kexec_stack
 /* Our assembly helper, in kexec_stub.S */
 extern NORET_TYPE void kexec_sequence(void *newstack, unsigned long start,
                                         void *image, void *control,
-                                        void (*clear_all)(void)) ATTRIB_NORET;
+                                        void (*clear_all)(void),
+                                        unsigned long kdump_flag) ATTRIB_NORET;
 
 /* too late to fail here */
 void default_machine_kexec(struct kimage *image)
 {
+        unsigned long kdump_flag = 0;
+
         /* prepare control code if any */
 
         /*
@@ -270,8 +273,10 @@ void default_machine_kexec(struct kimage *image)
         * using debugger IPI.
         */
 
-       if (crashing_cpu == -1)
-               kexec_prepare_cpus();
+        if (crashing_cpu == -1)
+                kexec_prepare_cpus();
+        else
+                kdump_flag = KDUMP_SIGNATURE;
 
         /* switch to a staticly allocated stack.  Based on irq stack code.
          * XXX: the task struct will likely be invalid once we do the copy!
@@ -284,7 +289,7 @@ void default_machine_kexec(struct kimage *image)
          */
         kexec_sequence(&kexec_stack, image->start, image,
                         page_address(image->control_code_page),
-                        ppc_md.hpte_clear_all);
+                        ppc_md.hpte_clear_all, kdump_flag);
         /* NOTREACHED */
 }