Re: Etherboot problems

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Simon Kagstrom wrote:
> At Thu, 09 Nov 2006 19:49:05 +0200,
> Avi Kivity wrote:
>   
>> Very likely this is due to VT's inability to virtualize real mode 
>> correctly.  Is anyone in the mood to extend x86_emulate.c to support 
>> real mode?
>>     
>
> I found what caused the problems for the etherboot startup and it now
> works fine (well, I've not tested downloading anything over the
> network, but at least it gets that far). I've started grub as well,
> and that works as before even with this fix.
>
> However, I'm not sure what the correct solution to the problem is.
>
> The crash occurs after etherboot has switched to protected mode and
> does a long jump to the new code segment:
>
> 	/* Load GDT */
>    1:	DATA32 lgdt %ds:(r2p_gdt-1b)(%bx)	/* Load GDT */
>
> 	/* Switch to protected mode */
>    2:	movl	%cr0, %eax
>    3:	orb	$CR0_PE, %al
>    4:	movl	%eax, %cr0
>
> 	/* flush prefetch queue, and reload %cs:%eip */
>    5:	DATA32 ljmp %ds:*(r2p_paddr-1b)(%bx)
> 	.code32
>
> The problem is that KVM zeroes all segment registers directly on a
> switch to protected mode (i.e., on instruction 4 above) - and %ds
> being zero causes a crash on the ljmp above.
>
> What I've done is to remove the zeroing of all segment registers
> except %ss, which is shown in the trivial patch below. I don't
> remember enough of transfer-to-protected-mode assembly to know the
> correct solution to this (the %ss zeroing needs to be kept for
> example, not sure why).
>   

VT does a lot of checks when entering guest mode.  These checks are 
stricter than what the processor can handle, and stricter than what some 
software expects.  The upshot is that virtualizing real mode and the 
transition between modes can't be made to work 100%.

The long term solution is to emulate rather than virtualize while in 
these states.  Short term, I'll see if there's some workaround we can 
apply here (your patch probably makes other guests fail; I'll check this 
though).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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