Re: [kvm-devel] [PATCH] gfxboot VMX workaround v2

[Anthony L. <an...@co...>]

Alexander Graf wrote:
>
> On Apr 7, 2008, at 6:05 PM, Anthony Liguori wrote:
>
>> Alexander Graf wrote:
>>> Hi,
>>>
>>> this is an improved version of the patch I sent several weeks ago to
>>> this list. Functionally nothing changed; it still hacks into gfxboot 
>>> and
>>> patches it to work on Intel CPUs on the fly. The big difference is that
>>> this version is cleaned up and should work with every future CPU 
>>> available.
>>>
>>> Please do _not_ apply this patch. I send it to the list only for
>>> interested people, who would like to have a working version of KVM for
>>> their systems right now. It is neither a proper fix nor the right
>>> approach to deal with this issue. It is merely a hack that works for me
>>> and maybe for others too.
>>>
>>
>> Perhaps a viable way to fix this upstream would be to catch the 
>> vmentry failure, look to see if SS.CPL != CS.CPL, and if so, invoke 
>> x86_emulate() in a loop until SS.CPL == CS.CPL.
>>
>> There are very few instructions in gfxboot that would need to be 
>> added to x86_emulate (if they aren't already there).
>
> In a previous thread Avi already explained a quite reasonable way to 
> approach this problem, which I believe is a really good approach. He 
> wanted to x86_emulate until the environment is "VMX friendly" again, 
> thus resolving big real mode problems as well.

I've got a slightly lamer approach than what Avi probably wants.  I lost 
interest in updating x86_emulate once I realized how far xen's copy has 
gotten.  To get GFXBOOT 3.3.28 working just requires adding far jmp to 
x86_emulate.  The sequence should look like:

        jmp pm_seg.prog_c32:switch_to_pm_20
switch_to_pm_20:

        bits 32

        mov ax,pm_seg.prog_d16
        mov ds,ax
        mov eax,ss

Which means we'll get 3 vmentry failures.  The two moves should already 
be supported by x86_emulate but I haven't confirmed.  It's not a 
complete solution to our real mode woes but I think it's a reasonable 
first step.

Regards,

Anthony Liguori
> I personally agree that the real approach is way superior to my patch. 
> I just won't have the time to do it in the near future and not being 
> able to boot intuitively hurts KVM users unnecessarily ;-).
>
> Regards,
>
> Alex