[PATCH] x86-64: Relocatable Kernel Support

This patch modifies the x86_64 kernel so that it can be loaded and run
at any 2M aligned address, below 512G.  The technique used is to
compile the decompressor with -fPIC and modify it so the decompressor
is fully relocatable.  For the main kernel the page tables are
modified so the kernel remains at the same virtual address.  In
addition a variable phys_base is kept that holds the physical address
the kernel is loaded at.  __pa_symbol is modified to add that when
we take the address of a kernel symbol.

When loaded with a normal bootloader the decompressor will decompress
the kernel to 2M and it will run there.  This both ensures the
relocation code is always working, and makes it easier to use 2M
pages for the kernel and the cpu.

AK: changed to not make RELOCATABLE default in Kconfig

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/asm-x86_64/page.h b/include/asm-x86_64/page.h
index 4974433bbf34..40a24d0df090 100644
--- a/include/asm-x86_64/page.h
+++ b/include/asm-x86_64/page.h
@@ -61,6 +61,8 @@ typedef struct { unsigned long pgd; } pgd_t;
 
 typedef struct { unsigned long pgprot; } pgprot_t;
 
+extern unsigned long phys_base;
+
 #define pte_val(x)      ((x).pte)
 #define pmd_val(x)      ((x).pmd)
 #define pud_val(x)      ((x).pud)
@@ -101,14 +103,14 @@ typedef struct { unsigned long pgprot; } pgprot_t;
 #define PAGE_OFFSET             __PAGE_OFFSET
 
 /* Note: __pa(&symbol_visible_to_c) should be always replaced with __pa_symbol.
-   Otherwise you risk miscompilation. */ 
+   Otherwise you risk miscompilation. */
 #define __pa(x)                 ((unsigned long)(x) - PAGE_OFFSET)
 /* __pa_symbol should be used for C visible symbols.
    This seems to be the official gcc blessed way to do such arithmetic. */ 
 #define __pa_symbol(x)          \
         ({unsigned long v;  \
           asm("" : "=r" (v) : "0" (x)); \
-          (v - __START_KERNEL_map); })
+          ((v - __START_KERNEL_map) + phys_base); })
 
 #define __va(x)                 ((void *)((unsigned long)(x)+PAGE_OFFSET))
 #ifdef CONFIG_FLATMEM