Laszlo Ersek <lersek@xxxxxxxxxx> writes: ... > Yes. > >> Without EPT, you don't >> hit the processor limitation with your setup, but the user should nevertheless >> still be notified. > > I disagree. > >> In fact, I think shadow paging code should also emulate >> this behavior if the gpa is out of range. > > I disagree. > > There is no "out of range" gpa. QEMU allocates enough memory, and it > should be completely transparent to the guest. The fact that it silently > breaks with nested paging if the host processor doesn't have enough > address bits is a bug (maybe a hardware bug, maybe a KVM bug; I'm not > sure, but I suspect it's a hardware bug). In any case the guest > shouldn't care at all. It is a *virtual* machine, and the VMM should lie > to it plausibly enough. How much RAM, and how many phys address bits the > host has, is a performance question, but it should not be a correctness > question. A 256 GB guest should run (slowly, but correctly) on a laptop > that has only 4 GB of RAM and only 36 phys addr bits, but plenty of swap > space. > > Because otherwise your argument could be extrapolated as "TCG should > break too if the gpa is 'out of range'". > > So, I disagree. Whatever memory you give to the guest should just work > (unless of course you want to emulate a small address width for the > *VCPU*, but that's absolutely not the use case here). What we have here > is a leaky abstraction: a PCPU limitation giving away a lie that the > guest should never notice. The guest should be able to use all memory > that was specified with QEMU's -m, regardless of TCG vs. KVM-without-EPT > vs. KVM-with-EPT. If the last case cannot work (due to hardware > limitations), that's fine, but then (and only then) a warning should be > printed. Hmm... Ok, I understand your point. So, this is more like a EPT limitation/bug in that Qemu isn't complaining about the memory assigned to the guest but EPT code is breaking owing to the processor physical address width. And honestly, I now think that this patch just makes the whole situation more confusing :) I am wondering if it's just possible for kvm to simply throw an error like a EPT misconfiguration or something .. Or in other words, if using a hardware assisted mechanism is just not possible, KVM will simply not let it run instead of letting a guest stuck in boot. > ... In any case, please understand that I'm not campaigning for this > warning :) IIRC the warning was your (very welcome!) idea after I > reported the problem; I'm just trying to ensure that the warning match > the exact issue I encountered. > > Thanks! > Laszlo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
