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SubjectRe: Regression: boot failure on AMD Elan TS-5500
Sean Young wrote:
> On Mon, Jun 16, 2008 at 04:19:44PM +0000, Sean Young wrote:
>
>> On Mon, Jun 16, 2008 at 06:37:12AM -0700, Jeremy Fitzhardinge wrote:
>>
>>> Rusty Russell wrote:
>>>
>>>> On Monday 16 June 2008 22:11:39 Sean Young wrote:
>>>>
>>>>
>>>>> The symptons are either a crash or reboot on booting the kernel. No
>>>>> printk's have occurred yet -- even with early printk on.
>>>>>
>>>>> 2.6.15 worked on this board however current does not. I've bisected it to:
>>>>>
>>>>> commit a24e785111a32ccb7cebafd24b1b1cb474ea8e5d
>>>>> Author: Rusty Russell <rusty@rustcorp.com.au>
>>>>> Date: Sun Oct 21 16:41:35 2007 -0700
>>>>>
>>>>> i386: paravirt boot sequence
>>>>>
>>>>>
>>>> Hi Sean,
>>>>
>>>> Thanks for tracking this down. Can we try reverting this in pieces to
>>>> see exactly what the cause was?
>>>>
>>>> 1) Revert arch/x86/boot/compressed/head_32.S
>>>> 2) If that doesn't fix it, Try removing the 8 lines which were added to
>>>> arch/x86/kernel/head_32.S
>>>>
>>>>
>>> And the arch/x86/boot/compressed/misc_32.c change too, just in case...
>>>
>> Reverting only head_32.S makes the problem go away. I didn't try any other
>> changes.
>>
>
> Looking at the beginning of startup_32, it seems ds is used before it is set:
>
> startup_32:
> cld
> /* test KEEP_SEGMENTS flag to see if the bootloader is asking
> * us to not reload segments */
> testb $(1<<6), BP_loadflags(%esi)
> jnz 1f
>
> cli
> movl $(__BOOT_DS),%eax
> movl %eax,%ds
> movl %eax,%es
> movl %eax,%fs
> movl %eax,%gs
> movl %eax,%ss
> 1:
>
> Since the testb instruction is a dereference, ds is implicitly used. If
> I move the testb to after "movl %eax,%ds" it seems to work (not that it
> would make any sense there, but just to prove the point).
>
> 1) Am I barking up the wrong tree?
>
> 2) If I'm right I have no idea what the correct solution is; it seems that
> a chicken & egg issue is introduced.
>
> Please advise. I am very new to all of this.

It's a bit odd that the boot loader neglected to set up ds properly, but
changing the testb line to

testb $(1<<6), %cs:BP_loadflags(%esi)

should work. (Or perhaps a %ss: override would be better?)

I'm assuming that the GDT setup isn't completely mad and that the
segments have the same base at least.

J


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