linux-kernel - Re: [patch 3/3] PTP: add kvm PTP driver

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Date:   Wed, 18 Jan 2017 15:02:23 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [patch 3/3] PTP: add kvm PTP driver



On 18/01/2017 14:36, Miroslav Lichvar wrote:
> On Wed, Jan 18, 2017 at 01:46:58PM +0100, Paolo Bonzini wrote:
>> On 18/01/2017 13:24, Marcelo Tosatti wrote:
>>>> Testcase: run a guest and a loop sending SIGUSR1 to vcpu0 (emulating
>>>> intense interrupts). Follows results:
> 
>>>> Do you still want to drop it in favour of simplicity?
> 
>> It's just that it's not obvious why you get better results with biased
>> host timestamps.  What makes the biased host timestamp more precise?
>>
>> I'd rather use PTP_SYS_OFFSET_PRECISE instead, but unfortunately chrony
>> does not support it---but I would still prefer you to support
>> PTP_SYS_OFFSET_PRECISE as well.
> 
> Interesting. I wasn't aware that there is a new ioctl for measuring
> the HW-sys offset. Adding support to chrony shouldn't be difficult.
> 
> If I understand it correctly, PTP_SYS_OFFSET can be emulated on top of
> PTP_SYS_OFFSET_PRECISE simply by copying the sys_realtime and device
> fields to corresponding ts slots. The apparent delay will be zero, but
> that's ok if the conversion is really accurate.

Yes, for 1 sample only.  Otherwise you'd have the same issue as in
Marcelo's driver (the device aka guest timestamp from
PTP_SYS_OFFSET_PRECISE would not be halfway between the system aka host
timestamps), and your idea below could be applied.

> I'm not sure if trying to do that in the opposite direction is a good
> idea. An application using PTP_SYS_OFFSET_PRECISE may assume the
> conversion is accurate and not include any delay/dispersion in an
> estimate of the maximum error, which is needed in NTP for instance.
> 
> If we know the host timestamp ts[1] is not in the middle between the
> guests timestamps ts[0] and ts[2], but rather closer to ts[2], why not
> simply shift ts[1] by (ts[2]-ts[0])/2 ?

Interesting idea!  For this to work, KVM needs to implement
getcrosstimestamp and ptp_chardev.c can then add an alternative
implementation of PTP_SYS_OFFSET, based on precise cross timestamps.

Something like

                for (i = 0; i <= sysoff->n_samples; i++) {
			// ... call getcrosststamp ...
			sysns = ktime_to_ns(xtstamp.sys_realtime);
			if (i > 0) {
				devns = ktime_to_ns(xtstamp.device);
				devns -= (sysns - prev_sysns) / 2;
				devts = ns_to_timespec(devns);
				pct->sec = devts.tv_sec;
				pct->nsec = devts.tv_nsec;
	                        pct++;
			}
			systs = ns_to_timespec(sysns);
                        pct->sec = ts.tv_sec;
                        pct->nsec = ts.tv_nsec;
                        pct++;
			prev_sysns = sysns;
                }

Marcelo, can you give it a try?

Thanks,

Paolo