Re: [patch 3/3] PTP: add kvm PTP driver

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Jan 16, 2017 at 05:54:11PM +0100, Radim Krcmar wrote:
> 2017-01-16 17:26+0100, Radim Krcmar:
> > 2017-01-13 15:40-0200, Marcelo Tosatti:
> >> On Fri, Jan 13, 2017 at 04:56:58PM +0100, Radim Krcmar wrote:
> >> > 2017-01-13 10:01-0200, Marcelo Tosatti:
> >>> > +		version = pvclock_read_begin(src);
> >>> > +
> >>> > +		ret = kvm_hypercall2(KVM_HC_CLOCK_OFFSET,
> >>> > +				     clock_off_gpa,
> >>> > +				     KVM_CLOCK_OFFSET_WALLCLOCK);
> >>> > +		if (ret != 0) {
> >>> > +			pr_err("clock offset hypercall ret %lu\n", ret);
> >>> > +			spin_unlock(&kvm_ptp_lock);
> >>> > +			preempt_enable_notrace();
> >>> > +			return -EOPNOTSUPP;
> >>> > +		}
> >>> > +
> >>> > +		tspec.tv_sec = clock_off.sec;
> >>> > +		tspec.tv_nsec = clock_off.nsec;
> >>> > +
> >>> > +		delta = rdtsc_ordered() - clock_off.tsc;
> >>> > +
> >>> > +		offset = pvclock_scale_delta(delta, src->tsc_to_system_mul,
> >>> > +					     src->tsc_shift);
> >>> > +
> >>> > +	} while (pvclock_read_retry(src, version));
> >>> > +
> >>> > +	preempt_enable_notrace();
> >>> > +
> >>> > +	tspec.tv_nsec = tspec.tv_nsec + offset;
> >>> > +
> >>> > +	spin_unlock(&kvm_ptp_lock);
> >>> > +
> >>> > +	if (tspec.tv_nsec >= NSEC_PER_SEC) {
> >>> > +		u64 secs = tspec.tv_nsec;
> >>> > +
> >>> > +		tspec.tv_nsec = do_div(secs, NSEC_PER_SEC);
> >>> > +		tspec.tv_sec += secs;
> >>> > +	}
> >>> > +
> >>> > +	memcpy(ts, &tspec, sizeof(struct timespec64));
> >>> 
> >>> But the whole idea is of improving the time by reading tsc a bit later
> >>> is just weird ... why is it better to provide
> >>> 
> >>>   tsc + x, time + tsc_delta_to_time(x)
> >>> 
> >>> than just
> >>> 
> >>>  tsc, time
> >>> 
> >>> ?
> >> 
> >> Because you want to calculate the value of the host realtime clock 
> >> at the moment of ptp_kvm_gettime.
> >> 
> >> We do:
> >> 
> >> 	1. kvm_hypercall.
> >> 	2. get {sec, nsec, guest_tsc}.
> >> 	3. kvm_hypercall returns.
> >> 	4. delay = rdtsc() - guest_tsc.
> >> 
> >> Where delay is the delta (measured with the TSC) between points 2 and 4.
> > 
> > I see now ... the PTP interface is just not good for our purposes.
> 
> There is getcrosststamp() callback in PTP, which seems to be exactly
> what we want when pairing with TSC, so the pvclock delay fixup can be
> dropped when using it.

What pvclock delay fixup you refer to? The "rdtsc() - clock_offset.tsc"
part? You can't drop it, because if you do then your "host realtime
clock read" will be behind by "rdtsc() - clock_offset.tsc" TSC cycles.
We want the highest precision as possible.