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

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Message-ID: <1168c0c1-fe82-c60f-6a15-9aa890b2b617@redhat.com>
Date:   Wed, 18 Jan 2017 17:07:11 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     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 16:54, Miroslav Lichvar wrote:
>> Sure, I'll check if it makes sense to implement PTP_SYS_OFFSET_PRECISE for 
>> KVM case.
> But is it really so precise that the application can safely assume
> there are no errors due to asymmetric delays, etc? I think
> PTP_SYS_OFFSET_PRECISE should be supported only if the accuracy of
> the offset measured between the HW and system clock is not worse than
> the precision of the system clock (typically few tens of nanoseconds).
> 
> It would be good to verify the accuracy of the offset when the host
> and guest clocks are synchronised to each other over PTP using two
> NICs with HW timestamping.

PTP_SYS_OFFSET_PRECISE works if the guest can compute its own timestamp
based on the same source as the device.  On bare metal you have:

- the source for system clock is the TSC (with clocksource_tsc)
- device provides a (time, ART) tuple

You can convert ART->TSC and then ask clocksource_tsc for a system time
based on the device-provided ART value.  Likewise for KVM:

- the source for system clock is the guest TSC (with kvmclock)
- host can provide a (time, guest TSC) tuple

The PTP driver can take the host-provided guest TSC, and ask kvmclock
for a system time based on that TSC value.  It's even simpler because
there's no ART->TSC conversion involved, and it will always be precise
and independent of any vmexit or interrupt delay.

Paolo