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SubjectRe: Regression: boot failure on AMD Elan TS-5500
Jeremy Fitzhardinge wrote:
>>
>> 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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