On Tue, Dec 08, 2020 at 06:25:13PM +0200, Maxim Levitsky wrote: > On Tue, 2020-12-08 at 17:02 +0100, Thomas Gleixner wrote: > > On Tue, Dec 08 2020 at 16:50, Maxim Levitsky wrote: > > > On Mon, 2020-12-07 at 20:29 -0300, Marcelo Tosatti wrote: > > > > > +This ioctl allows to reconstruct the guest's IA32_TSC and TSC_ADJUST value > > > > > +from the state obtained in the past by KVM_GET_TSC_STATE on the same vCPU. > > > > > + > > > > > +If 'KVM_TSC_STATE_TIMESTAMP_VALID' is set in flags, > > > > > +KVM will adjust the guest TSC value by the time that passed since the moment > > > > > +CLOCK_REALTIME timestamp was saved in the struct and current value of > > > > > +CLOCK_REALTIME, and set the guest's TSC to the new value. > > > > > > > > This introduces the wraparound bug in Linux timekeeping, doesnt it? > > > > Which bug? > > > > > It does. > > > Could you prepare a reproducer for this bug so I get a better idea about > > > what are you talking about? > > > > > > I assume you need very long (like days worth) jump to trigger this bug > > > and for such case we can either work around it in qemu / kernel > > > or fix it in the guest kernel and I strongly prefer the latter. > > > > > > Thomas, what do you think about it? > > > > For one I have no idea which bug you are talking about and if the bug is > > caused by the VMM then why would you "fix" it in the guest kernel. > > The "bug" is that if VMM moves a hardware time counter (tsc or anything else) > forward by large enough value in one go, > then the guest kernel will supposingly have an overflow in the time code. > I don't consider this to be a buggy VMM behavior, but rather a kernel > bug that should be fixed (if this bug actually exists) It exists. > Purely in theory this can even happen on real hardware if for example SMM handler > blocks a CPU from running for a long duration, or hardware debugging > interface does, or some other hardware transparent sleep mechanism kicks in > and blocks a CPU from running. > (We do handle this gracefully for S3/S4) > > > > > Aside of that I think I made it pretty clear what the right thing to do > > is. > > This is orthogonal to this issue of the 'bug'. > Here we are not talking about per-vcpu TSC offsets, something that I said > that I do agree with you that it would be very nice to get rid of. > > We are talking about the fact that TSC can jump forward by arbitrary large > value if the migration took arbitrary amount of time, which > (assuming that the bug is real) can crash the guest kernel. QE reproduced it. > This will happen even if we use per VM global tsc offset. > > So what do you think? > > Best regards, > Maxim Levitsky > > > > > Thanks, > > > > tglx > > >
