Please read https://en.wikipedia.org/wiki/Posting_style or related documents on email quoting. 2016-02-23 07:11+0000, Avi Cohen: >> 2016-02-21 16:57+0000, Avi Cohen: >>> Hello, >>> >>> Last week I've sent a mail regarding the kvm-clock accuracy. >>> Now I try to draw-up my question again, any answer/partial/hint is >>> greatly appreciated >>> >>> Our application is running in a Tenant's Virtual Machine in a data-centre. >>> We have some OAM functions running in the VMs. >>> One OAM function is to measure one-way delay between VMa and VMb. >>> One way delay measurement requires that all machines should be synchronized to a common central clock. >>> Accuracy requirement is in order of 10s nano-seconds, hence only the 1588v2/PTP is suitable here. >>> Since we cannot use HW timestamping in a virtual machine (we cannot force using SR-IOV), I thought to run PTP on the physical machines and to sync the VMs to the host by the kvm-clock. >> >> kvmclock doesn't do synchronization with host clock or UTC. >> kvmclock bases on host's notion of *passed* time. >> kvmclock allows the guest to measure a flow of time. >> >> It is another layer's job to translate kvmclock result into a timestamp that can be compared. kvmclock was designed like that, because KVM wants to make a guest independent on hosts. > > I see the system time written by KVM whenever the VM is entered - in kvm_guest_time_update() > > How can the guest (or another layer's job ) - translate kvmclock result into a timestamp that can be compared ? If the layer has a good idea about the number of ticks the second takes on both clocks, then the layer is synchronized with a (time on one clock, time on second clock) pair. You get a timestamp comparable with the other clock by providing a timestamp from one of synchronized clocks. The hard part is getting the pair. (A duration of the second is pretty stable and the pair needs to be renewed on a change anyway.) >> > In Theory - Is it possible to achieve 10s ns accuracy between VM clock and the host clock ? >> >> It is. > > How ? this is the only question Look at last paragraph of my previous email. (That one has 1 ns accuracy if KVM is using the masterclock.) Note that it is impossible to confirm the accuracy with your measurement because delay between sending and receiving a packet is normally *far* longer that 100 ns. >> > its system_time and the VM_TSC @ current time to the pvclock page , then the guest OS can calculate its current time by: >> >> KVM doesn't write its (= host's) system_time. >> KVM writes *guest's* system_time. Guest's system_time at VM_TSC. >> >> (system_time is 0 when the VM starts. sytem_time can store ~584 years worth of nanoseconds, but using an arbitrary offset makes everything simpler. This part of kvmclock is pretty confusing, so system_time is likely the source of misunderstanding.) >> >> Have you read that kvmclock does synchronization with host time somewhere? >> > Yes - see - in vcpu_enter_guest() there is a call to kvm_guest_time_update() > > Which update the pvclock paget for the guest - see that it updates the system_time with the host system-time I think you misunderstand the code, see kvmclock_offset. system_time is incremented by a second if the host thinks that a second has passed since last update, but that doesn't allow the guest to tell host time. -- 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
