Re: [Xen-devel] [PATCH v2 6/6] x86/time: implement PVCLOCK_TSC_STABLE_BIT

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

>> The main
>> difference I see between both would be the base system time: 
>> read_platform_stime
>> uses stime_platform_stamp as base, and computes a difference from the
>> read_counter (i.e. rdtsc() ) with previously saved platform-wide stamp
>> (platform_timer_stamp). get_s_time uses the stime_local_stamp (updated from
>> stime_master_stamp on local_time_calibration) as base plus delta from 
>> rdtsc()
>> with local_tsc_stamp. And since this is now all TSC, and TSC monotonically
>> increase and is synchronized across CPUs, both calls would end up returning 
>> the
>> same or a always up-to-date value, whether cpu_time have a larger gap or not
>> from stime_platform_stamp. Unless the concern you are raising comes from the
>> fact CPU 0 calibrates much sooner than the last calibrated CPU, as opposed 
>> to
>> roughly at the same time with std_rendezvous?
> 
> In a way, yes. I'm concerned by the two time stamps no longer
> being obtained at (almost) the same time. If that's not having
> any bad consequences, the better.

I don't think there would be bad consequences as both timestamps correspond to
the same time reference - thus returning always the latest system time
irrespective of the gap between both stamps.

If you prefer I can go back with my initial approach (v1, with std_rendezvous)
to have both timestamps closely updated. And later (post-release?) revisit the
introduction of nop_rendezvous. Perhaps this way is more reasonable?

Joao

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