Re: [PATCH v3 2/6] KVM: x86: switch to masterclock update using timekeeper functionality

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/08/2017 11:30, Denis Plotnikov wrote:
>> - implementing kvm_clock_read_with_cycles (can be merged with patch 6)
>
> Having a stable clocksource for kvmklock means making kvmclock stable.
> The patch enables this functionality that's why I'd prefer to keep patch
> 6 separate

Ok, though it depends on how you deal with cs_stable.

>> - using ktime_get_snapshot in KVM (can be merged with patch 4?)
>
> agree, but want to keep 4 separate. Just to make the changes done
> logically consecutive in git tree.

Fine by me.

>> Maybe what we want is some kind of "bool cycles_valid", and then
>> read_clock_and_systime can return it:
>>
>>
>>         if (clock->read_clock_and_systime) {
>>             systime_snapshot->cycles_valid =
>>              clock->read_clock_and_systime(
>>                 &now, &systime_snapshot->cycles);
>>         } else {
>>             now = tk_clock_read(&tk->tkr_mono);
>>             systime_snapshot->cycles_valid = true;
>>             systime_snapshot->cycles = now;
>>         }
>>
>> ?  (This is honestly just a suggestion, which may be wrong depedning
>> on the answer to the two questions above).
>
> cs_stable means "there is no unexpected time jumps".

But even for kvmclock there are no unexpected time jumps, the global
accumulator in pvclock_clocksource_read ensures that.  And the concept
is different from CLOCK_SOURCE_UNSTABLE which will basically blacklist
the clocksource for hrtimers.

It seems a different concept to me, somewhat specific to
ktime_get_snapshot.  More precisely, the question is "is there a 1:1
mapping from cycles to nanoseconds?"---but if there is no such mapping
cycles is useless, hence my proposal of "cycles_valid".

Thanks,

Paolo

> I don't mind merging this "check stability" functionality with
> read_clock_and_systime. Actually, I thought about having it there but
> eventually split it to make the code more explicit
> (detailed and understandable).
> I'd like to use your approach but keep the variable name the same.
> 
> Thanks for reviewing!
> 
> Denis
>>
>> Paolo
>>
>>>           systime_snapshot->cs_was_changed_seq = tk->cs_was_changed_seq;
>>>           systime_snapshot->clock_was_set_seq = tk->clock_was_set_seq;
>>>           base_real = ktime_add(tk->tkr_mono.base,
>>>                         tk_core.timekeeper.offs_real);
>>>           base_raw = tk->tkr_raw.base;
>>> +        base_boot = ktime_add(tk->tkr_mono.base,
>>> +                      tk_core.timekeeper.offs_boot);
>>>           nsec_real = timekeeping_cycles_to_ns(&tk->tkr_mono, now);
>>>           nsec_raw  = timekeeping_cycles_to_ns(&tk->tkr_raw, now);
>>>       } while (read_seqcount_retry(&tk_core.seq, seq));
>>>   -    systime_snapshot->cycles = now;
>>>       systime_snapshot->real = ktime_add_ns(base_real, nsec_real);
>>>       systime_snapshot->raw = ktime_add_ns(base_raw, nsec_raw);
>>> +    systime_snapshot->boot = ktime_add_ns(base_boot, nsec_real);
>>>   }
>>>   EXPORT_SYMBOL_GPL(ktime_get_snapshot);
>>>  
>>
>