Re: Regression: boot failure on AMD Elan TS-5500

[H. Peter Anvin]

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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