Re: [RFC] ELF Relocatable x86 and x86_64 bzImages

From: Vivek Goyal
Date: Fri Aug 04 2006 - 18:54:31 EST

On Tue, Aug 01, 2006 at 04:58:49AM -0600, Eric W. Biederman wrote:
> The problem:
> We can't always run the kernel at 1MB or 2MB, and so people who need
> different addresses must build multiple kernels. The bzImage format
> can't even represent loading a kernel at other than it's default address.
> With kexec on panic now starting to be used by distros having a kernel
> not running at the default load address is starting to become common.
Hi Eric,

There seems to be a small anomaly in the current set of patches for i386.

For example if one compiles the kernel with CONFIG_RELOCATABLE=y
and CONFIG_PHYSICAL_START=0x400000 (4MB) and he uses grub to load
the kernel then kernel would run from 1MB location. I think user would
expect it to run from 4MB location.

I think distro's might want to keep above config options enabled.
CONFIG_RELOCATABLE=y so that kexec can load kdump kernel at a
different address and CONFIG_PHYSICAL_START=non 1MB location, to
extract better performance. (As we had discussions on mailing list
some time back.)

In principle this is a limitation on boot-loaders part but as we can
not fix the boot-loaders out there, probably we can try fixing it
at kernel level.

What I have done here is that decompressor code will determine the
final resting place of the kernel based on boot loader type. So
if I have been loaded by kexec, I am supposed to run from loaded address
otherwise I am supposed to run from CONFIG_PHYSICAL_START as I have been
loaded at 1MB address due to boot loader limitation and that's not the

A prototype patch is attached with the mail. I have assumed that I can
assign a boot loader type id 9 to kexec (Documentation/i386/boot.txt).
Also assuming that all other boot loaders apart from kexec have got 1MB
limitation. If not, its trivial to include their boot loader ids also.

I have tested this patch and it works fine. What do you think about
this approach ?


Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>

arch/i386/boot/compressed/head.S | 32 ++++++++++++++++++++++++++++++--
1 file changed, 30 insertions(+), 2 deletions(-)

diff -puN arch/i386/boot/compressed/head.S~debug1-patch arch/i386/boot/compressed/head.S
--- linux-2.6.18-rc3-1M/arch/i386/boot/compressed/head.S~debug1-patch 2006-08-04 18:03:02.000000000 -0400
+++ linux-2.6.18-rc3-1M-root/arch/i386/boot/compressed/head.S 2006-08-04 18:18:26.000000000 -0400
@@ -60,13 +60,32 @@ startup_32:
* a relocatable kernel this is the delta to our load address otherwise
* this is the delta to CONFIG_PHYSICAL start.
+ /* If loaded by non kexec boot loader, then we will be loaded
+ * at 1MB fixed address. But probably the intention is to run
+ * from a address for which kernel has been compiled which can
+ * be non 1MB.
+ */
+ xorl %eax, %eax
+ movb 0x210(%esi), %al
+ / * check boot loader type. Kexec bootloader id 9, version any */
+ shrl $4, %eax
+ subl $0x9, %eax
+ jnz 1f
+ /* Run kernel from the location it has been loaded at. */
movl %ebp, %ebx
+ jmp 2f
+ /* Run the kernel from compiled destination location. */
+1: movl $(CONFIG_PHYSICAL_START - startup32), %ebx
movl $(CONFIG_PHYSICAL_START - startup_32), %ebx

- /* Replace the compressed data size with the uncompressed size */
+2: /* Replace the compressed data size with the uncompressed size */
subl input_len(%ebp), %ebx
movl output_len(%ebp), %eax
addl %eax, %ebx
@@ -95,7 +114,16 @@ startup_32:
/* Compute the kernel start address.
+ xorl %eax, %eax
+ movb 0x210(%esi), %al
+ / * check boot loader type. Kexec bootloader id 9, version any */
+ shrl $4, %eax
+ subl $0x9, %eax
+ jnz 3f
leal startup_32(%ebp), %ebp
+ jmp 4f
@@ -103,7 +131,7 @@ startup_32:
* Jump to the relocated address.
- leal relocated(%ebx), %eax
+4: leal relocated(%ebx), %eax
jmp *%eax
.section ".text"
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