Jeremy Fitzhardinge wrote:
quoted text
>>
>> 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.
> 

This should have been set up by the *boot code* (specifically lines 
57-61 of arch/x86/boot/pmjump.S) since he's using a conventional boot 
loader (syslinux) so something is utterly fuggled up.  Using %cs: here 
should be safe, though (and *is* more conservative, after all, why 
otherwise bother reloading these segments at all?), but it still 
concerns me a great deal if this is broken in this way.  It's definitely 
better than %ss:.

In particular, I'm wondering if the Elan CPU has any strange ordering 
requirements with regards to the protected mode transition that we're 
not obeying.

It would be interesting to put in a heavyweight "brutally synchronizing" 
instruction like WBINVD at various places in arch/x86/boot/pmjump.S and 
see if it helps.

	-hpa
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