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Subject[PATCH] [20/22] x86_64: build-time checking
Date

From: Vivek Goyal <vgoyal@in.ibm.com>

o X86_64 kernel should run from 2MB aligned address for two reasons.
- Performance.
- For relocatable kernels, page tables are updated based on difference
between compile time address and load time physical address.
This difference should be multiple of 2MB as kernel text and data
is mapped using 2MB pages and PMD should be pointing to a 2MB
aligned address. Life is simpler if both compile time and load time
kernel addresses are 2MB aligned.

o Flag the error at compile time if one is trying to build a kernel which
does not meet alignment restrictions.

Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
arch/x86_64/boot/compressed/misc.c | 2 +-
arch/x86_64/kernel/head64.c | 7 +++++++
include/asm-x86_64/page.h | 1 +
3 files changed, 9 insertions(+), 1 deletion(-)
Index: linux/arch/x86_64/boot/compressed/misc.c
===================================================================
--- linux.orig/arch/x86_64/boot/compressed/misc.c
+++ linux/arch/x86_64/boot/compressed/misc.c
@@ -358,7 +358,7 @@ asmlinkage void decompress_kernel(void *
insize = input_len;
inptr = 0;

- if ((ulg)output & 0x1fffffUL)
+ if ((ulg)output & (__KERNEL_ALIGN - 1))
error("Destination address not 2M aligned");
if ((ulg)output >= 0xffffffffffUL)
error("Destination address too large");
Index: linux/arch/x86_64/kernel/head64.c
===================================================================
--- linux.orig/arch/x86_64/kernel/head64.c
+++ linux/arch/x86_64/kernel/head64.c
@@ -62,6 +62,13 @@ void __init x86_64_start_kernel(char * r
{
int i;

+ /*
+ * Make sure kernel is aligned to 2MB address. Catching it at compile
+ * time is better. Change your config file and compile the kernel
+ * for a 2MB aligned address (CONFIG_PHYSICAL_START)
+ */
+ BUILD_BUG_ON(CONFIG_PHYSICAL_START & (__KERNEL_ALIGN - 1));
+
/* clear bss before set_intr_gate with early_idt_handler */
clear_bss();

Index: linux/include/asm-x86_64/page.h
===================================================================
--- linux.orig/include/asm-x86_64/page.h
+++ linux/include/asm-x86_64/page.h
@@ -78,6 +78,7 @@ extern unsigned long phys_base;
#endif /* !__ASSEMBLY__ */

#define __PHYSICAL_START CONFIG_PHYSICAL_START
+#define __KERNEL_ALIGN 0x200000
#define __START_KERNEL (__START_KERNEL_map + __PHYSICAL_START)
#define __START_KERNEL_map 0xffffffff80000000
#define __PAGE_OFFSET 0xffff810000000000
-
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