2015-01-21 23:40-0200, Marcelo Tosatti: > On Wed, Jan 21, 2015 at 06:00:37PM +0100, Radim Krčmář wrote: > > 1) The bug happens because a guest expects greater precision. > > I consider that as a guest problem. kvmclock never guaranteed > > anything, so unmet expectations should be a recoverable error. > > delta = pvclock_data.tsc_timestamp - RDTSC > > Guest expects delta to be smaller than a given threshold. It does > not expect greater precision. > > Size of delta does not affect precision. I don't understand what the guest wants to achieve with the delta. I thought that checking this only made sense if the guest didn't believe that PV clock works with large delta. And they only want precision. (What else is there on a clock?) Disclaimer: I haven't read the code. (It wasn't in vanilla 2.6.16.) > > 2) With time, the probability that 2.6.16 is used is getting lower, > > while people looking at KVM's code appear. > > - At what point are we going to drop 2.6.16 support? > > (We shouldn't let mistakes drag us down forever ... > > Or are we dooming KVM on purpose?) > > One of the features of virtualization is to be able to run old > operating systems? True, I'll assign higher priority to it. > > 3) The patch made me ask more silly questions than it answered :) > > (Why can't other software depend on previous behavior? > > Documentation/virtual/kvm/msr.txt: > > whose data will be filled in by the hypervisor periodically. > Only one write, or registration, is needed for each VCPU. The interval > between updates of this structure is arbitrary and implementation-dependent. > The hypervisor may update this structure at any time it sees fit until > anything with bit0 == 0 is written to it. Exactly, this made me think it is not a KVM problem. (And I wondered why wouldn't we yield to other misuses of it.) > > > Supporting old guests is important. > > > > It comes at a price. > > (Mutually exclusive goals are important as well.) > > This phrase is awkward. Overlapping goals are negative, > then? (think of a large number of totally overlapping goals). Even if both mutually exclusive goals are positive, we can only choose one. (Sorry, I don't see the neccessity between overlapping goals and negativity.) -- 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
