> 2016-04-05 15:07+0000, Avi Cohen:
> > What do you think about this simple solution:
> > Currently the KVM updates the system-time in the structure
> > pvclock_vcpu_time_info in the guest memory with the MONOTONIC time
> struct pvclock_vcpu_time_info {
> > u32 version;
> > u32 pad0;
> > u64 tsc_timestamp;
> > u64 system_time;
> > u32 tsc_to_system_mul;
> > s8 tsc_shift;
> > u8 flags;
> > u8 pad[2];
> > } __attribute__((__packed__));
> >
> > This is the default kvm-clcok.
> > I think to add another clock source named - kvm-clock-realtime, and to update
> system_time with host's REAL_TIME clock.
> > What is your opinion ?
>
> 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.
>
Yes - it will be ok to update the pvclock_wall_clock with the host CLOCK_REALTIME.
But I think I also need the guest's TSC at that time of update,
struct pvclock_wall_clock {
37 u32 version;
38 u32 sec;
39 u32 nsec;
New field tsc_timestamp;
New field tsc_to_system_mul;
40 } __attribute__((__packed__));
Now the guest - when need to read the current time - can do something like this :
Current_time = sec * sec_to_nano + nsec + (rdtdc () - tsc_timestamp) * tsc_to_system_mul
> > 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
> - changes in wallclock should trigger guest notifiers as if the change
> was done by the guest
> - wallclock is updated only when the guest writes to the MSR, which
> would be wasteful for frequent reads and not possible in userspace
>
> We'll have enough experience to decide on followup if no solution using the
> existing framework is acceptable.
I will do some design work with existing framework and send it soon
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