* 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.
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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