Date: Fri, 28 Jun 2024 22:27:48 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Peter Hilber <peter.hilber@...nsynergy.com>, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 linux-rtc@...r.kernel.org, "Ridoux, Julien" <ridouxj@...zon.com>,
 virtio-dev@...ts.linux.dev, "Luu, Ryan" <rluu@...zon.com>
CC: "Christopher S. Hall" <christopher.s.hall@...el.com>,
 Jason Wang <jasowang@...hat.com>, John Stultz <jstultz@...gle.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
 Richard Cochran <richardcochran@...il.com>, Stephen Boyd <sboyd@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Alessandro Zummo <a.zummo@...ertech.it>,
 Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [RFC PATCH v2] ptp: Add vDSO-style vmclock support

On 28 June 2024 17:38:15 BST, Peter Hilber <peter.hilber@...nsynergy.com> wrote:
>On 28.06.24 14:15, David Woodhouse wrote:
>> On Fri, 2024-06-28 at 13:33 +0200, Peter Hilber wrote:
>>> On 27.06.24 16:52, David Woodhouse wrote:
>>>> I already added a flags field, so this might look something like:
>>>>
>>>>         /*
>>>>          * Smearing flags. The UTC clock exposed through this structure
>>>>          * is only ever true UTC, but a guest operating system may
>>>>          * choose to offer a monotonic smeared clock to its users. This
>>>>          * merely offers a hint about what kind of smearing to perform,
>>>>          * for consistency with systems in the nearby environment.
>>>>          */
>>>> #define VMCLOCK_FLAGS_SMEAR_UTC_SLS (1<<5) /* draft-kuhn-leapsecond-00.txt */
>>>>
>>>> (UTC-SLS is probably a bad example but are there formal definitions for
>>>> anything else?)
>>>
>>> I think it could also be more generic, like flags for linear smearing,
>>> cosine smearing(?), and smear_start_sec and smear_end_sec fields (relative
>>> to the leap second start). That could also represent UTC-SLS, and
>>> noon-to-noon, and it would be well-defined.
>>>
>>> This should reduce the likelihood that the guest doesn't know the smearing
>>> variant.
>> 
>> I'm wary of making it too generic. That would seem to encourage a
>> *proliferation* of false "UTC-like" clocks.
>> 
>> It's bad enough that we do smearing at all, let alone that we don't
>> have a single definition of how to do it.
>> 
>> I made the smearing hint a full uint8_t instead of using bits in flags,
>> in the end. That gives us a full 255 ways of lying to users about what
>> the time is, so we're unlikely to run out. And it's easy enough to add
>> a new VMCLOCK_SMEARING_XXX type to the 'registry' for any new methods
>> that get invented.
>> 
>> 
>
>My concern is that the registry update may come after a driver has already
>been implemented, so that it may be hard to ensure that the smearing which
>has been chosen is actually implemented.

Well yes, but why in the name of all that is holy would anyone want to invent *new* ways to lie to users about the time? If we capture the existing ones as we write this, surely it's a good thing that there's a barrier to entry for adding more?


>But the error bounds could be large or missing. I am trying to address use
>cases where the host steps or slews the clock as well.

The host is absolutely intended to be skewing the clock to keep it accurate as the frequency of the underlying oscillator changes, and the seq_count field will change every time the host does so.

Do we need to handle steps differently? Or just let the guest deal with it?

If an NTP server suddenly steps the time it reports, what does the client do?

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