> From: Radim Krčmář [mailto:rkrcmar@xxxxxxxxxx] > Sent: Tuesday, 05 April, 2016 11:25 PM > To: Avi Cohen > Cc: kvm@xxxxxxxxxxxxxxx > Subject: Re: kvm-clock again > Would you be ok with a wall_time entry in pvclock_vcpu_time_info that would > tell the host real time at system_time? > struct pvclock_wall_clock should say the host real time when system_time was > 0, so it's the same. wallclock = CLOCK_REALTIME, most likely. > > Creating a new clock would allow us to get rid of many other problems, but your > goal might be achievable with existing interfaces. > > > I want to enter the KVM code and try to make a REAL_TIME and accurate > synchronization between the host and guest clocks . > > Currently I don’t see that KVM updates the guest’s clock when I manually set > the host’s clock. I don’t see any impact on the guest’s clock Also after booting > the guest I see a diff-time of ~1ms between the 2 clocks. > > Questions: > > - What can you tell about the feasibility of this task ? > > Doable. > > > - Can you give me a reference/guides for this task’s design ? where to start ? > potential problems ? pvclock ? when to update , when reentering the VM ? etc.. > > Start by using kvm_get_wallclock to pass the host CLOCK_REALTIME into the > guest and turn wallclock into guest CLOCK_REALTIME. > > A reasonable solution would be to create a thread that periodically synchronizes > CLOCK_REALTIME with wallclock. The wallclock thread would behave like > PTP/NTP, so it should be easy to do. > (A notification from the host that the wallclock has changed would be better, > but needs new interface.) > > I think that using wallclock directly as CLOCK_REALTIME would cause more > complications: > - many guest interfaces expect that they can touch CLOCK_REALTIME, but > that shouldn't be allowed Do you mean simultaneously guests write to MSR ? Guests running on different physical CPU's can simultaneously write to the MSR and this should trigger kvm update of wall-clock - correct ? I understand that guest writing to MSR does not necessarily trigger VM-exit - correct ? > - changes in wallclock should trigger guest notifiers as if the change > was done by the guest I don’t understand what do u mean here. if for example the host's clock was changed because of PTP time-set, then after some time a guest is writing to MSR to get host real-time clock , I expect that the new wall clock update will include this time-set. > - wallclock is updated only when the guest writes to the MSR, which > would be wasteful for frequent reads and not possible in userspace > As you suggest - this will be performed by a new kernel thread which should only write to the MSR and exit. i.e. it will not look into the wall-clock structure. Only when user application is requesting the current time (clock_gettime) - this call (clock_gettime or a another new call) will check the content of the pvclock_wall_clock and calculate the current time. > We'll have enough experience to decide on followup if no solution using the > existing framework is acceptable. ��.n��������+%������w��{.n�����o�^n�r������&��z�ޗ�zf���h���~����������_��+v���)ߣ�
