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

[Jeremy Fitzhardinge]

Sean Young wrote:
> On Mon, Jun 30, 2008 at 03:42:47PM -0700, H. Peter Anvin wrote:
>   
> > Jeremy Fitzhardinge wrote:
> >     
> > > Maybe it really does require the far jump immediately after setting PE 
> > > in cr0...
> > >
> > > Hm, I don't remember this paragraph being in vol 3a, section 8.9.1 
> > > before.  Is it a recent addition?
> > >
> > >   Random failures can occur if other instructions exist between steps
> > >   3 and 4 above.  Failures will be readily seen in some situations,
> > >   such as when instructions that reference memory are inserted between
> > >   steps 3 and 4 while in system management mode.
> > >
> > >       
> > I don't remember that, either.
> >     
>
> Which document are we talking about? 
>   

*The* document ;)
http://www.intel.com/products/processor/manuals/ specifically, Volume 
3a: System Programming Guide, Part 1. 

Section 8.9.1 describes the steps needed to turn on protected mode 
correctly.  It says that you need to do a far jump or call immediately 
after turning on protected mode.  Linux has not done it immediately, and 
there has been a school of thought that this advice is a workaround for 
some obsolete CPU, and is not something we have to worry about now.

However, the paragraph I quoted was added since the previous release of 
the manual, and so presumably documents a current concern.  
Specifically, the mention of SMM is interesting, because I gather that 
embedded-class processors like the Elan are very SMM-dependent.

> > Sean: could you try the following patch?
> >
> > 	-hpa
> >     
>
>   
> > diff --git a/arch/x86/boot/pmjump.S b/arch/x86/boot/pmjump.S
> > index ab049d4..141b6e2 100644
> > --- a/arch/x86/boot/pmjump.S
> > +++ b/arch/x86/boot/pmjump.S
> > @@ -33,6 +33,8 @@ protected_mode_jump:
> >  	movw	%cs, %bx
> >  	shll	$4, %ebx
> >  	addl	%ebx, 2f
> > +	jmp	1f			# Short jump to serialize on 386/486
> > +1:
> >  
> >  	movw	$__BOOT_DS, %cx
> >  	movw	$__BOOT_TSS, %di
> > @@ -40,8 +42,6 @@ protected_mode_jump:
> >  	movl	%cr0, %edx
> >  	orb	$X86_CR0_PE, %dl	# Protected mode
> >  	movl	%edx, %cr0
> > -	jmp	1f			# Short jump to serialize on 386/486
> > -1:
> >  
> >  	# Transition to 32-bit mode
> >  	.byte	0x66, 0xea		# ljmpl opcode
> >     
>
> I'm afraid it doesn't work. Maybe I can find something in the AMD Elan
> documentation. Would a fence make sense?
>   

Not really, but if it fixes the bug it won't hurt anyone else (unless 
older processors treat it as an illegal instruction).

   J
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