Messages in this thread | | | Date | Mon, 13 Apr 2009 06:16:25 +0200 | From | Ingo Molnar <> | Subject | Re: [tip:x86/setup] x86, setup: "glove box" BIOS calls -- infrastructure |
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* Avi Kivity <avi@redhat.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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