> > - if (vcpu->arch.hv_clock.flags & PVCLOCK_TSC_STABLE_BIT) {
> > - u64 tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> > - ns = __pvclock_read_cycles(&vcpu->arch.hv_clock, tsc);
> > - } else {
> > - ns = ktime_get_boot_ns() + ka->kvmclock_offset;
> > - }
>
> If we access the "global" master clock, it would be better to prevent it
> from changing under our hands with
> spin_lock(&ka->pvclock_gtod_sync_lock).
Yes, good idea.
> > + if (!ka->use_master_clock)
> > + return ktime_get_boot_ns() + ka->kvmclock_offset;
> >
> > - return ns;
> > + hv_clock.tsc_timestamp = ka->master_cycle_now;
> > + hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
> > + kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
> > + &hv_clock.tsc_shift,
> > + &hv_clock.tsc_to_system_mul);
>
> Doesn't this result in a minor drift with scaled clock, because the
> guest can be combining two systems that approximate frequency?
You mean instead of doing read_l1_tsc?
> 1) tsc_shift and tsc_to_system_mul for kvmclock scaling
> 2) hardware TSC scaling ratio
>
> If we are on a 7654321 kHz TSC and TSC-ratio scale to 1234567 kHz and
> then tsc_shift+tsc_to_system_mul kvmclock-scale to 1000000 kHz, we
> should be using multipliers of
> 0.161290204578564186163606151349022336533834941074459772460... and
> 0.810000591300431649315104000025920018921613812778083328000...,
> to achieve that. Those multipliers cannot be precisely expressed in
> what we have (shifts and 64/32 bit multipliers with intermediate values
> only up to 128 bits), so performing the scaling will result in slightly
> incorrect frequency.
>
> The result of combining two operations that alter the freqency is quite
> unlikely to cancel out and produce the same result as an operation that
> uses a different shift+multiplier to scale in one step, so I think that
> we aren't getting the same time as the guest with TSC-scaling is seeing.
I think you get pretty good precision, since 30 fractional bits are more
or less equivalent to nanosecond precision. For example, cutting the two
ratios above to 30 fractional bits I get respectively 173184038/2^30
and 869731512/2^30. Multiplying them gives 140279173/2^30 which matches
exactly the fixed point representation of 1000000/7654321.
Since the TSC scaling frequency has a larger precision (32 or 48 bits),
you should get at most 1 ulp error, which is not bad.
Paolo
> (I'd be happier if we didn't ignore this drift when the whole endeavor
> started just to get rid of a drift, but introducing a minor bug is still
> improving the situation -- I'm ok with first two changes only.)
>
> > + return __pvclock_read_cycles(&hv_clock, rdtsc());
> > }
> >
> > u64 get_kvmclock_ns(struct kvm *kvm)
> > --
> > 1.8.3.1
> >
>
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