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

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Date:	Mon, 13 Apr 2009 09:44:59 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Pavel Machek <pavel@....cz>,
	mingo@...hat.com, linux-kernel@...r.kernel.org, tglx@...utronix.de,
	hpa@...ux.intel.com, rjw@...k.pl, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/setup] x86, setup: "glove box" BIOS calls --	infrastructure

Ingo Molnar wrote:
> * Avi Kivity <avi@...hat.com> 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.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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