Re: [tip:x86/setup] x86, setup: "glove box" BIOS calls -- infrastructure

[Avi Kivity]

Ingo Molnar wrote:
> * Avi Kivity <avi@xxxxxxxxxx> wrote:
>
>   
> > Ingo Molnar wrote:
> >     
> > > > Sure, go ahead and wrap them in some kind of "save and restore all  
> > > > registers" wrapping, but nothing fancier than that. It would just be 
> > > > overkill, and likely to break more than it fixes.
> > > >     
> > > >         
> > > Yeah. I only brought up the virtualization thing as a 
> > > hypothetical: "if" corrupting the main OS ever became a 
> > > widespread problem. Then i made the argument that this is 
> > > unlikely to happen, because Windows will be affected by it just 
> > > as much. (while register state corruptions might go unnoticed 
> > > much more easily, just via the random call-environment clobbering 
> > > of registers by Windows itself.)
> > >
> > > The only case where i could see virtualization to be useful is 
> > > the low memory RAM corruption pattern that some people have 
> > > observed.
> > >       
> > You could easily check that by checksumming pages (or actually 
> > copying them to high memory) before the call, and verifying after 
> > the call.
> >     
>
> Yes, we could do memory checks, and ... hey, we already do that:
>
>    bb577f9: x86: add periodic corruption check
>    5394f80: x86: check for and defend against BIOS memory corruption
>
> ... and i seem to be the one who implemented it! ;-)
>
> That check resulted in logs showing the BIOS corrupting Linux memory 
> across s2ram cycles or HDMI plug/unplug events on certain boxes (are 
> Hollywood rootkits in the BIOS now?), and resulted in some 
> head-scratching but not much more.
>   

Then there's definitely no point in putting it into a container, is 
there?  I mean, we could track down the exact instruction which caused 
the corruption, but how would it help us?

> > I don't think the effort is worth the benefit in this case, but 
> > there actually is an interesting use case for this.  SMM is known 
> > to be harmful to deterministic replay games and to real time 
> > response.  If we can virtualize SMM, we can increase the range of 
> > hardware on which the real time kernel is able to deliver real 
> > time guarantees.
> >     
>
> Hey, i do have a real sweet spot for deterministic execution - but 
> SMM, while not problem-free (like most of firmware), also has a very 
> real role in not letting various hardware melt. So SMM should be 
> thought of as a flexible extended arm of hardware - not some sw bit.
>
> So i think that the memory of that SMM virtualization chapter you've 
> almost read should be quickly erased from your mind. (Via forceful 
> means if prompt corrective self-action is not forthcoming.)
>   

I'll keep my remaining neurons, thanks.


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