Hi Christoffer, On 25.06.2015 10:04, Christoffer Dall wrote: > On Wed, Jun 24, 2015 at 03:54:57PM +0100, Marc Zyngier wrote: >> Userspace is allowed to set the guest's view of CNTVCT, which turns >> into setting CNTVOFF for the whole VM. One thing userspace is not supposed >> to do is to update that register while the guest is running. Time will >> either move forward (best case) or backward (really bad idea). Either way, >> this shouldn't happen. >> >> This patch prevents userspace from touching CNTVOFF as soon as a vcpu >> has been started. This ensures that time will keep monotonically increase. >> >> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx> >> --- >> >> QEMU seems to trigger this at boot time, and I have no idea why it does so. >> It would be good to find out, hence the RFC tag. > > Is this at kernel boot time you see this, or at system startup time? > > IIRC, QEMU creates a throw-away VM with the default CPU target time, > reads out all the system registers to get the KVM reset values of those, > then creates the real VM, and feeds back in all the system register > reset values, as a method for QEMU and KVM to be in sync about the reset > state of the machine. If we do this, and include CNTVCT, then that > would probably trigger this, but the VCPU really shouldn't have been run > at that time... > > We should prevent userspace from fiddling with this register post VCPU > start regardless, but yes, it would be good to find out why this is > happening in the first place. > > How did you notice this and does it manifest itself in some user-visible > ugliness? > > Thanks, > -Christoffer > You can read the whole history here: https://groups.google.com/forum/#!topic/osv-dev/2w101csH65E It causes clock-related bugs with time jumping backward when relying on the virtual counter register in the guest, whenever a cpu is booted (primary, secondary via PSCI), and actually whenever the monitor is used to stop, info registers etc. Once the VM is created, I think QEMU should not request kvm to change the virtual offset of the VM anymore: maybe an unexpected consequence of QEMU's target-arm/kvm64.c::kvm_arch_put_registers ? Thanks, Claudio -- 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
