On Mon, 29 Oct 2007 01:05:58 +0100 (MET), Mikael Pettersson wrote:
>My old 486 fails to boot with the 2.6.24-rc1 kernel.
>Grub loads it, 4 lines of text appear but not the kernel's
>"Linux version <blah> greet", and the machine reboots.
>Double-checked with a serial console: nothing appears
>before it reboots.
>
>All 2.6 kernels up to 2.6.23 worked fine on this machine.
Problem identified and solved.
The last good kernel was 2.6.23-git17. The change in 2.6.23-git18
that broke my 486 was a24e785111a32ccb7cebafd24b1b1cb474ea8e5d,
which included the following:
>--- a/arch/x86/boot/compressed/head_32.S
>+++ b/arch/x86/boot/compressed/head_32.S
>@@ -27,13 +27,22 @@
> #include <asm/segment.h>
> #include <asm/page.h>
> #include <asm/boot.h>
>+#include <asm/asm-offsets.h>
>
> .section ".text.head","ax",@progbits
> .globl startup_32
>
> startup_32:
>- cld
>- cli
>+ /* check to see if KEEP_SEGMENTS flag is meaningful */
>+ cmpw $0x207, BP_version(%esi)
>+ jb 1f
>+
>+ /* test KEEP_SEGMENTS flag to see if the bootloader is asking
>+ * us to not reload segments */
>+ testb $(1<<6), BP_loadflags(%esi)
>+ jnz 2f
>+
>+1: cli
> movl $(__BOOT_DS),%eax
> movl %eax,%ds
> movl %eax,%es
That is, it added loads from %ds:offset(%esi) before the code
that sanitises %ds et al. Commenting out these loads and tests
made the kernel boot again.
Adding debugging code to head_32.S and decompress_kernel(),
I found that startup_32 actually is invoked with a good %ds
(== 0x18 == __BOOT_DS) and a flat 4GB data segment in the
corresponding GDT entry.
Since data accesses work after %ds is (re)loaded with __BOOT_DS,
it's clear that on this machine, the hidden registers behind %ds
are stale on entry to startup_32. After adding a patch to reload
%ds with itself (not __BOOT_DS) at startup_32 the kernel doesn't
crash and reboot any more on this machine.
The machine in question is a ca 1993 vintage Siemens 486 with
a Quadtel S3 / Phoenix BIOS from 1994, booting via grub-0.95-13
from Fedora Core 4.
Signed-off-by: Mikael Pettersson <[email protected]>
---
arch/x86/boot/compressed/head_32.S | 5 +++++
1 files changed, 5 insertions(+)
--- linux-2.6.24-rc1-git13/arch/x86/boot/compressed/head_32.S.~1~ 2007-11-04 16:34:33.000000000 +0100
+++ linux-2.6.24-rc1-git13/arch/x86/boot/compressed/head_32.S 2007-11-04 16:44:15.000000000 +0100
@@ -33,6 +33,11 @@
.globl startup_32
startup_32:
+ /* workaround for BIOSen or boot loaders that don't reload %ds
+ after changing the GDT (insane but unfortunately true) */
+ movl %ds,%eax
+ movl %eax,%ds
+
cld
/* test KEEP_SEGMENTS flag to see if the bootloader is asking
* us to not reload segments */
-
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