Hi Sheng, thanks for your thoughts!
Yang, Sheng wrote: > On Thursday 11 September 2008 23:11:14 Jan Kiszka wrote: >> Hi Sheng, >> >> we had parts of this discussion privately a while back, but now I need >> to dig deeper. I finally have to complete and push out user space NMI >> injection that bitrots over and over again in my queue. >> >> Intel CPUs with virtual NMI support can easily be programmed to inject >> NMIs only when the guest CPU is ready or tell the guest to exit as soon >> as it changes its NMI readiness. But we are also facing a lot of CPUs >> that were produced without this feature, and we need to find some way to >> achieve virtual NMI support for them, even if the interruptibility of >> the guest is not as good as with true virtual NMIs. >> >> Now my questions to you or anyone else with VMX expert knowledge: >> >> The System Programming Guide, Volume 3B, 27.2 says that "Without >> NMI-window exiting support, the VMM will need to poll and check the >> interruptibility state of the guest to deliver virtual NMIs." >> Interruptibility involves, besides "blocked by MOV SS" and "blocked by >> STI", the NMI nesting. So, how can the VMM track if the guest is still >> inside its NMI handler after event injection? > > I think, we have no direct idea about that... > >> According to 22.6.1, NMI blocking is active as long as the guest runs if >> the "NMI-exiting" control bit is 1 (and setting it appears to be >> required to avoid that the guest can block NMIs for the host, right?). > > Yes. > >> But what does this mean for bit 3 of the interruptibility state? Can I >> still use it after some guest exit for whatever reason (like a hard IRQ) >> to poll if the guest can now accept pending virtual NMIs? > > No. Though the public spec is a little ambiguous, I just checked some > document > and found that, if "virtual NMI" is not enabled, "Block by NMI" just > indicated block *host physical* NMI rather than virtual NMI. Grmbl. > >> In that case, virtual NMI injections would only be delayed a bit on older >> CPUs, but could still work reliably, right? Or is there some other way to >> track the NMI interruptibility on such CPUs? > > I hope so, but sadly seems we can't know if we can inject NMI to guest > without > enable "virtual NMI". > > After some brainstorming with my colleague Haitao, we found one way *may* can > work on non-virtual nmi supported hardware, but it's very tricky and > untested(just a theoretical idea), also sacrifice host physical NMI, and > depends on guest NMI implement. > > The basic idea is: > 1. Disable "NMI Exiting" feature, so that guest would handle any host > physical > NMI. > 2. Set "Blocking by NMI" to true. So if guest execute "iret", this bit should > be cleaned. This also block physical NMIs. > 3. Enable "IRQ Window" and ensure IDT vector 2 is a interrupt gate(so that IF > bit is cleared when executing the handler). So we use IRQ window to replace > NMI window here, otherwise for the period between IRET and VMEXIT, host > physical NMIs would be handled by guest handler. > 4. Check "IRQ Window" exit, if "Blocking by NMI" is cleared(also STI and SS), > we can inject NMI. > > It's just untested idea, and I think it's quite tricky. If you still have > interest, you may try it, but I can't guarantee the result... Well, I thought in this direction already as well. But I wasn't sure if, while the guest is in NMI context, hard IRQs will also be blocked and won't cause guest exists anymore. Can you comment on this? However, even if that is no issue, I do not really like this workaround. Specifically the need to fiddle with the guest's IDT and, of course, that we may delays host NMIs. I'm now playing with this idea, basically a "light" version of yours: After we injected an NMI, consider the guest being in NMI context until the next IRQ window opens. That may cause lost NMIs if the guest blocks IRQ delivery infinitely, but I would say this is rather untypical and still much better than the current situation (no NMIs at all!). And it is easier to implement. Comments? Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
