Message-ID: <4869558A.9020907@goop.org>
Date: Mon, 30 Jun 2008 14:52:10 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Sean Young <sean@mess.org>
CC: Rusty Russell <rusty@rustcorp.com.au>, linux-kernel@vger.kernel.org,
       "H. Anvin" <hpa@zytor.com>
Subject: Re: Regression: boot failure on AMD Elan TS-5500
References: <20080616121138.GA51097@atlantis.8hz.com> <200806162327.34912.rusty@rustcorp.com.au> <48566C88.6000702@goop.org> <20080616161944.GA53539@atlantis.8hz.com> <20080630214347.GA57988@atlantis.8hz.com>
In-Reply-To: <20080630214347.GA57988@atlantis.8hz.com>
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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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