Re: kvm-clock again

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-04-06 13:51+0000, Avi Cohen:
>> 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

No need.  pvclock_wall_clock.{sec,nsec} is synchronized with system_time
and system_time is synchronized with TSC.  To get the current real time,
you just add times from pvclock_vcpu_time_info, pvclock_wall_clock and
TSC, like kvm_get_wallclock() does.