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

[Radim Krcmar <rkrcmar@redhat.com>]

2017-01-16 15:08-0200, Marcelo Tosatti:
> 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?

Yes.

>       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.

The TSC read will be some cycles old when the hypercall ends, but that
doesn't matter, because we will pass {sec, nsec, guest_tsc} to PTP and
PTP should plug them into kernel's realtime clock roughly like this:

  sec/nsec + (rdtsc() - guest_tsc) * tsc_freq

Adding delay to guest_tsc and sec/nsec cannot improve precision.
(And will likely degrade it as kvmclock's frequency is incorrect.)

> We want the highest precision as possible.

I agree, which is why we don't want to lose precision in the delay
guesswork because of gettime64().