Date:	Mon, 13 Apr 2009 06:16:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
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


* 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.

See:

    "corrupt PMD after resume"
 
    http://bugzilla.kernel.org/show_bug.cgi?id=11237

>> The problem with it, it happens on s2ram transitions, and that is 
>> driven by SMM mainly - which is a hypervisor sitting on top of 
>> all the other would-be-hypervisors and thus not virtualizable.
>
> AMD in fact has a chapter called "Containerizing Platform SMM" or 
> words to the effect, which describes how to take a running system 
> and drop its SMM mode into a virtualization container.  I made a 
> point of skipping over those pages with my eyes closed so I can't 
> tell you how incredibly complex it is.
>
> It's probably even doable on Intel, though much more difficult, 
> due to Intel not supporting big real mode in a guest, and most SMM 
> code using it to access memory.  You'd end up running most of the 
> code in the emulator, and performing the transitions by hand.
>
> Of course, the VMM has to be careful not to trigger SMM itself, or 
> much merriment ensues.
>
>> Which leaves us without a single practical case. So it's not 
>> going to happen.
>
> 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.)

The determinism issue can IMHO be solved via a simpler measure: by 
making sure the owner of the box always knows when SMMs happened. 
Real-time folks are very picky about their hardware and there's many 
suppliers, so it would have a real market effect. I know about one 
case where a BIOS was modified to lessen its SMM latency impact.

	Ingo
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