Message-ID: <f6940954-334a-458b-af32-f03d8efbe607@opensynergy.com>
Date: Mon, 11 Mar 2024 19:24:33 +0100
From: Peter Hilber <peter.hilber@...nsynergy.com>
To: David Woodhouse <dwmw2@...radead.org>,
 Peter Hilber <peter.hilber@...nsynergy.com>, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux.dev, virtio-dev@...ts.oasis-open.org,
 linux-arm-kernel@...ts.infradead.org, linux-rtc@...r.kernel.org,
 "virtio-comment@...ts.oasis-open.org" <virtio-comment@...ts.oasis-open.org>
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>,
 "Ridoux, Julien" <ridouxj@...zon.com>
Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

On 08.03.24 13:33, David Woodhouse wrote:
> On Fri, 2024-03-08 at 11:32 +0100, Peter Hilber wrote:
>> On 07.03.24 15:02, David Woodhouse wrote:
>>> On Mon, 2023-12-18 at 08:38 +0100, Peter Hilber wrote:
>>>> RFC v3 updates
>>>> --------------
>>>>
>>>> This series implements a driver for a virtio-rtc device conforming to
>>>> spec
>>>> RFC v3 [1]. It now includes an RTC class driver with alarm, in
>>>> addition to
>>>> the PTP clock driver already present before.
>>>>
>>>> This patch series depends on the patch series "treewide: Use
>>>> clocksource id
>>>> for get_device_system_crosststamp()" [3]. Pull [4] to get the combined
>>>> series on top of mainline.
>>>>
>>>> Overview
>>>> --------
>>>>
>>>> This patch series adds the virtio_rtc module, and related bugfixes.
>>>> The
>>>> virtio_rtc module implements a driver compatible with the proposed
>>>> Virtio
>>>> RTC device specification [1]. The Virtio RTC (Real Time Clock) device
>>>> provides information about current time. The device can provide
>>>> different
>>>> clocks, e.g. for the UTC or TAI time standards, or for physical time
>>>> elapsed since some past epoch.
>>>
>>> Hm, should we allow UTC? If you tell me the time in UTC, then
>>> (sometimes) I still don't actually know what the time is, because some
>>> UTC seconds occur twice. UTC only makes sense if you provide the TAI
>>> offset, surely? Should the virtio_rtc specification make it mandatory
>>> to provide such?
>>>
>>> Otherwise you're just designing it to allow crappy hypervisors to
>>> expose incomplete information.
>>>
>>
>> Hi David,
>>
>> (adding virtio-comment@...ts.oasis-open.org for spec discussion),
>>
>> thank you for your insightful comments. I think I take a broadly similar
>> view. The reason why the current spec and driver is like this is that I
>> took a pragmatic approach at first and only included features which work
>> out-of-the-box for the current Linux ecosystem.
>>
>> The current virtio_rtc features work similar to ptp_kvm, and therefore
>> can
>> work out-of-the-box with time sync daemons such as chrony.
>>
>> As of RFC spec v3, UTC clock only is allowed. If mandating a TAI clock
>> as
>> well, I am afraid that
>>
>> - in some (embedded) scenarios, the TAI clock may not be available
>>
>> - crappy hypervisors will pass off the UTC clock as the TAI clock.
>>
>> For the same reasons, I am also not sure about adding a *mandatory* TAI
>> offset to each readout. I don't know user-space software which would
>> leverage this already (at least not through the PTP clock interface).
>> And
>> why would such software not go straight for the TAI clock instead?
>>
>> How about adding a requirement to the spec that the virtio-rtc device
>> SHOULD expose the TAI clock whenever it is available - would this
>> address
>> your concerns?
>
> I think that would be too easy for implementors to miss, or decide not
> to obey. Or to get *wrong*, by exposing a TAI clock but actually
> putting UTC in it.
>
> I think I prefer to mandate the tai_offset field with the UTC clock.
> Crappy implementations will just set it to zero, but at least that
> gives a clear signal to the guests that it's *their* problem to
> resolve.

To me there are some open questions regarding how this would work. Is there
a use case for this with the v3 clock reading methods, or would it be
enough to address this with the Virtio timekeeper?

Looking at clock_adjtime(2), the tai_offset could be exposed, but probably
best alongside some additional information about leap seconds. I am not
aware about any user-space user. In addition, leap second smearing should
also be addressed.

>
>
>
>
>>>> PTP clock interface
>>>> -------------------
>>>>
>>>> virtio_rtc exposes clocks as PTP clocks to userspace, similar to
>>>> ptp_kvm.
>>>> If both the Virtio RTC device and this driver have special support for
>>>> the
>>>> current clocksource, time synchronization programs can use
>>>> cross-timestamping using ioctl PTP_SYS_OFFSET_PRECISE2 aka
>>>> PTP_SYS_OFFSET_PRECISE. Similar to ptp_kvm, system time
>>>> synchronization
>>>> with single-digit ns precision is possible with a quiescent reference
>>>> clock
>>>> (from the Virtio RTC device). This works even when the Virtio device
>>>> response is slow compared to ptp_kvm hypercalls.
>>>
>>> Is PTP the right mechanism for this? As I understand it, PTP is a way
>>> to precisely synchronize one clock with another. But in the case of
>>> virt guests synchronizing against the host, it isn't really *another*
>>> clock. It really is the *same* underlying clock. As the host clock
>>> varies with temperature, for example, so does the guest clock. The only
>>> difference is an offset and (on x86 perhaps) a mathematical scaling of
>>> the frequency.
>>>
>>> I was looking at this another way, when I came across this virtio-rtc
>>> work.
>>>
>>> My idea was just for the hypervisor to expose its own timekeeping
>>> information — the counter/TSC value and TAI time at a given moment,
>>> frequency of the counter, and the precision of both that frequency
>>> (±PPM) and the TAI timestamp (±µs).
>>>
>>> By putting that in a host/guest shared data structure with a seqcount
>>> for lockless updates, we can update it as time synchronization on the
>>> host is refined, and we can even cleanly handle live migration where
>>> the guest ends up on a completely different host. It allows for use
>>> cases which *really* care (e.g. timestamping financial transactions) to
>>> ensure that there is never even a moment of getting *wrong* timestamps
>>> if they haven't yet resynced after a migration.
>>
>> I considered a similar approach as well, but integrating that with the
>> kernel timekeeping seemed too much effort for the first step. However,
>> reading the clock from user space would be much simpler.
>
> Right. In fact my *first* use case was userspace, specifically in the
> context of https://github.com/aws/clock-bound — but anything we design
> for this absolutely has to be usable for kernel timekeeping too.
>
> It's also critical to solve the Live Migration problem.
>
> But is it so hard to integrate into the kernel timekeeping? My plan
> would have given us effectively an infinite number of cross-reads of
> the realtime clock vs. TSC. You don't have to actually read from a
> virtio device; you just read the TSC and do the maths, using the values
> in the shared memory region. Couldn't that be used to present a PTP
> device to the guest kernel just the same as you do at the moment?

Yes, and it would also decrease the clock reading overhead (saving at least
the Virtio response interrupt and associated scheduling). Would make sense
to me.

To be clear, with "kernel timekeeping integration" I meant to have
timekeeping.c derive the time directly from the Virtio timekeeper.

>
> You could probably even simulate PPS with it. Typically with PPS we
> have to catch the hardware interrupt and then read the TSC as soon as
> possible thereafter. With this, you'd be able to *calculate* the TSC
> value at the start of the next second, and wouldn't have to suffer the
> real hardware latency :)
>
>>>
>>> Now I'm trying to work out if I should attempt to reconcile with your
>>> existing virtio-rtc work, or just decide that virtio-rtc isn't trying
>>> to solve the actual problem that we have, and go ahead with something
>>> different... ?
>>>
>>
>> We are certainly interested into the discussed, say, "virtual
>> timekeeper"
>> mechanism, which would also solve a lot of problems for us (especially
>> if
>> it would be integrated with kernel timekeeping). Even without Linux
>> kernel
>> timekeeping, the virtual timekeeper would be useful to us for guests
>> with
>> simpler timekeeping, and potentially for user space applications.
>>
>> Our current intent is to at first try to upstream the current (RFC spec
>> v3)
>> feature set. I think the virtual timekeeper would be suitable as an
>> optional feature of virtio_rtc (with Virtio, this could easily be added
>> after initial upstreaming). It is also possible to have a virtio-rtc
>> device
>> only implement the virtual timekeeper, but not the other clock reading
>> methods, if these are of no interest.
>
> Yeah, that might make sense. I was thinking of a simple ACPI/DT device
> exposing a page of memory and *maybe* an interrupt for when an update
> happens. (With the caveat that the interrupt would always occur too
> late by definition, so it's no substitute for using the seqlock
> correctly in applications that *really* care and are going to get fined
> millions of dollars for mis-timestamping their transactions.)
>
> But using the virtio-rtc device as the vehicle for that shared memory
> page is reasonable too. It's not even mutually exclusive; we could
> expose the *same* data structure in memory via whatever mechanisms we
> wanted.
>
> One other thing to note is I think we're being very naïve about the TSC
> on x86 hosts. Theoretically, the TSC for every vCPU might run at a
> different frequency, and even if they run at the same frequency they
> might be offset from each other. I'm happy to be naïve but I think we
> should be *explicitly* so, and just say for example that it's defined
> against vCPU0 so if other vCPUs are different then all bets are off.

ATM Virtio has no notion of vCPUs, or vCPU topology. So I wonder if you
have an opinion on how to represent this in a platform-independent way.

Thank you for the comments,

Peter

>
> We *can* cope with TSC frequencies changing. Fundamentally, that's the
> whole *point*; NTP calibrates itself as the underlying frequency does
> change due to temperature changes, etc. — so a deliberate frequency
> scaling, or even a live migration, are just a slightly special case of
> the same thing.
>
> One thing I have added to the memory region is a migration counter. In
> the ideal case, guests will be happy just to use the hypervisor's
> synchronization. But in some cases the guests may want to do NTP (or
> PPS, PTP or something else) for themselves, to have more precise
> timekeeping than the host. Even if the host is advertising itself as
> stratum 16, the guest still needs to know of *migration*, because it
> has to consider itself unsynchronized when that happens

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