[PATCH V2] kexec, x86: fix kexec when boot_params.hardware_subarch != 0

[Chris Leech]

kexec needs to keep the subarch setting the same as the running kernel
in the boot parameters, or the kernel will die in early setup.  I ran
into this with X86_SUBARCH_MRST, but it should apply to CE4100 and any
future subarch that uses non-default early setup code.

This patch requires debugfs mounted at /sys/kernel/debug, as that's
the only way I know of to get at the running kernels boot_params.
Without debugfs mounted it falls back to the current behavior of
assuming subarch 0.

Signed-off-by: Chris Leech <christopher.leech at linux.intel.com>
---
 kexec/arch/i386/x86-linux-setup.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/kexec/arch/i386/x86-linux-setup.c b/kexec/arch/i386/x86-linux-setup.c
index f843ca4..50b32be 100644
--- a/kexec/arch/i386/x86-linux-setup.c
+++ b/kexec/arch/i386/x86-linux-setup.c
@@ -20,6 +20,7 @@
 #include <string.h>
 #include <stdarg.h>
 #include <stdlib.h>
+#include <stddef.h>
 #include <errno.h>
 #include <limits.h>
 #include <sys/types.h>
@@ -396,12 +397,32 @@ out:
 		real_mode->eddbuf_entries);
 }
 
+#define BOOT_PARAMS_DBGFS	"/sys/kernel/debug/boot_params/data"
+
+void setup_subarch(struct x86_linux_param_header *real_mode)
+{
+	int data_file;
+	const off_t offset = offsetof(typeof(*real_mode), hardware_subarch);
+
+	data_file = open(BOOT_PARAMS_DBGFS, O_RDONLY);
+	if (data_file < 0)
+		return;
+	if (lseek(data_file, offset, SEEK_SET) < 0)
+		goto close;
+	read(data_file, &real_mode->hardware_subarch, sizeof(uint32_t));
+close:
+	close(data_file);
+}
+
 void setup_linux_system_parameters(struct x86_linux_param_header *real_mode,
 					unsigned long kexec_flags)
 {
 	/* Fill in information the BIOS would usually provide */
 	struct memory_range *range;
 	int i, ranges;
+
+	/* get subarch from running kernel */
+	setup_subarch(real_mode);
 	
 	/* Default screen size */
 	real_mode->orig_x = 0;