Attachments:

smime.p7s (5.83 kB)

2024-03-11 18:48:55

by Peter Hilber

[permalink] [raw]

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 [email protected] 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

2024-03-12 17:16:08

On Wed, 2024-03-13 at 10:45 +0100, Peter Hilber wrote:
> On 12.03.24 18:15, David Woodhouse wrote:
> > On Mon, 2024-03-11 at 19:24 +0100, Peter Hilber wrote:
> > > 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:
> > > > > > 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 [email protected] 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.
> > >
> >
> > Is there even a standard yet for leap-smearing? Will it be linear over
> > 1000 seconds like UTC-SLS? Or semi-raised-cosine over 24 hours, which I
> > think is what Google does? Meta does something different again, don't
> > they?
> >
> > Exposing UTC as the only clock reference is bad enough; when leap
> > seconds happen there's a whole second during which you don't *know*
> > which second it is. It seems odd to me, for a precision clock to be
> > deliberately ambiguous about what the time is!
>
> Just to be clear, the device can perfectly expose only a TAI reference
> clock (or both UTC and TAI), the spec is just completely open about this,
> as it tries to work for diverse use cases.

As long as the guest *knows* what it's getting, sure.

> >
> > But if the virtio-rtc clock is defined as UTC and then expose something
> > *different* in it, that's even worse. You potentially end up providing
> > inaccurate time for a whole *day* leading up to the leap second.
> >
> > I think you're right that leap second smearing should be addressed. At
> > the very least, by making it clear that the virtio-rtc clock which
> > advertises UTC shall be used *only* for UTC, never UTC-SLS or any other
> > yet-to-be-defined variant.
> >
>
> Agreed.
>
> > Please make it explicit that any hypervisor which wants to advertise a
> > smeared clock shall define a new type which specifies the precise
> > smearing algorithm and cannot be conflated with the one you're defining
> > here.
> >
>
> I will add a requirement that the UTC clock can never have smeared/smoothed
> leap seconds.

Thanks.

> I think that not every vendor would bother to first add a definition of a
> smearing algorithm. Also, I think in some cases knowing the precise
> smearing algorithm might not be important (when having the same time as the
> hypervisor is enough and accuracy w.r.t. actual time is less important).
>
> So maybe I should add a VIRTIO_RTC_CLOCK_UTC_SMEARED clock type, which for
> now could catch every UTC-like clock which smears/smoothes leap seconds,
> where the vendor cannot be bothered to add the smearing algorithm to spec
> and implementations.

Please $DEITY no.

Surely the whole point of this effort is to provide guests with precise
and *unambiguous* knowledge of what the time is?

Using UTC is bad enough, because for a UTC timestamp in the middle of a
leap second the guest can't know know *which* occurrence of that leap
second it is, so it might be wrong by a second. To resolve that
ambiguity needs a leap indicator and/or tai_offset field.

But if you allow and encourage the use of smeared time without even a
specification of *how* it's smeared... that's even worse. You have an
unknown inaccuracy of up to a second for whole periods of time around a
leap second. That's surely the *antithesis* of what we're trying to do
here? Without an actual definition of the smearing, how is a guest
actually supposed to know what time it is?

(I suppose you could add a tai_offset_nanoseconds field? I don't know
that I want to *encourage* that thought process...)

> As for UTC-SLS, this *could* also be added, although [1] says
>
> It is inappropriate to use Internet-Drafts as reference material or
> to cite them other than as "work in progress."
>
> [1] https://datatracker.ietf.org/doc/html/draft-kuhn-leapsecond-00
>
> > > > 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.
> >
> > Well, it doesn't have a notion of TSCs either; you include that by
> > implicit reference don't you?
>
> I think I can add a SHOULD requirement which vaguely refers to vCPU 0, or
> boot vCPU. But the Virtio device is not necessarily hosted by a hypervisor,
> so the device might not even know which vCPUs there are. E.g. there is even
> interest to make virtio-rtc work as part of the virtio-net device (which
> might be implemented in hardware).

Sure, but those implementations aren't going to offer the TSC pairing
at all, are they?

Attachments:

smime.p7s (5.83 kB)

2024-03-13 12:58:40

by Alexandre Belloni

[permalink] [raw]

Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

On 13/03/2024 12:29:38+0000, David Woodhouse wrote:
> On Wed, 2024-03-13 at 12:18 +0100, Alexandre Belloni wrote:
> >
> > I still don't know anything about virtio but under Linux, an RTC is
> > always UTC (or localtime when dual booting but let's not care) and never
> > accounts for leap seconds. Having an RTC and RTC driver behaving
> > differently would be super inconvenient. Why don't you leave this to
> > userspace?
>
> Well yes, we don't need to expose *anything* from the hypervisor and we
> can leave it all to guest userspace. We can run NTP on every single one
> of *hundreds* of guests, leaving them all to duplicate the work of
> calibrating the *same* underlying oscillator.
>

Really, I see the point of sharing the time accurately between the host
and the guest and not duplicating the effort. This is not what I am
objecting to.

> I thought we were trying to avoid that, by having the hypervisor tell
> them what the time was. If we're going to do that, we need it to be
> sufficiently precise (and some clients want to *know* the precision),
> and above all we need it to be *unambiguous*.
>
> If the hypervisor says that the time is 3692217600.001, then the guest
> doesn't actually know *which* 3692217600.001 it is, and thus it still
> doesn't know the time to an accuracy better than 1 second.
>

The RTC subsystem has a 1 second resolution and this is not going to
change because there is no point doing so for the hardware, to get the
time precisely, UIE MUST be used there is no vendor that will just
support reading the time/date or timestamp as this is way too imprecise.

> And if we start allowing the hypervisor to smear clocks in some other
> underspecified ways, then we end up with errors of up to 1 second in
> the clock for long periods of time *around* the leap second.
>
> We need to avoid that ambiguity.

Exactly my point and I said, reading an RTC is always UTC and never
handles leap seconds so if userspace doesn't handle the leap second and
updates the RTC, too bad. There are obviously no RTC that will smear
clock unless instructed to, so the hypervisor must not smear the clock.

Note that this is not an issue for the subsystem because if you are not
capable to track an accurate clock, the RTC itself will have drifted by
much more than a second in between leap second inclusions.

>
> > I guess I'm still questioning whether this is the correct interface to
> > expose the host system time instead of an actual RTC.
>
> If an RTC device is able to report '23:59:60' as the time of day, I
> suppose that *could* resolve the ambiguity. But talking to a device is
> slow; we want guests to be able to know the time — accurately — with a
> simple counter/tsc read and some arithmetic. Which means *paired* reads
> of 'RTC' and the counter, and a precise indication of the counter
> frequency.

23:59:60 is not and will never be allowed in the RTC subsystem as this
is an invalid value for the hardware.

The TSC or whatever CPU counter/clock that is used to keep the system
time is not an RTC, I don't get why it has to be exposed as such to the
guests. PTP is fine and precise, RTC is not.

--
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

2024-03-13 14:16:43

by Peter Hilber

[permalink] [raw]

Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

On 13.03.24 12:18, Alexandre Belloni wrote:
> On 13/03/2024 10:45:54+0100, Peter Hilber wrote:
>>> Exposing UTC as the only clock reference is bad enough; when leap
>>> seconds happen there's a whole second during which you don't *know*
>>> which second it is. It seems odd to me, for a precision clock to be
>>> deliberately ambiguous about what the time is!
>>
>> Just to be clear, the device can perfectly expose only a TAI reference
>> clock (or both UTC and TAI), the spec is just completely open about this,
>> as it tries to work for diverse use cases.
>>
>>>
>>> But if the virtio-rtc clock is defined as UTC and then expose something
>>> *different* in it, that's even worse. You potentially end up providing
>>> inaccurate time for a whole *day* leading up to the leap second.
>>>
>>> I think you're right that leap second smearing should be addressed. At
>>> the very least, by making it clear that the virtio-rtc clock which
>>> advertises UTC shall be used *only* for UTC, never UTC-SLS or any other
>>> yet-to-be-defined variant.
>>>
>>
>> Agreed.
>>
>>> Please make it explicit that any hypervisor which wants to advertise a
>>> smeared clock shall define a new type which specifies the precise
>>> smearing algorithm and cannot be conflated with the one you're defining
>>> here.
>>>
>>
>> I will add a requirement that the UTC clock can never have smeared/smoothed
>> leap seconds.
>>
>> I think that not every vendor would bother to first add a definition of a
>> smearing algorithm. Also, I think in some cases knowing the precise
>> smearing algorithm might not be important (when having the same time as the
>> hypervisor is enough and accuracy w.r.t. actual time is less important).
>>
>> So maybe I should add a VIRTIO_RTC_CLOCK_UTC_SMEARED clock type, which for
>> now could catch every UTC-like clock which smears/smoothes leap seconds,
>> where the vendor cannot be bothered to add the smearing algorithm to spec
>> and implementations.
>>
>
> I still don't know anything about virtio but under Linux, an RTC is
> always UTC (or localtime when dual booting but let's not care) and never
> accounts for leap seconds. Having an RTC and RTC driver behaving
> differently would be super inconvenient. Why don't you leave this to
> userspace?
>
> I guess I'm still questioning whether this is the correct interface to
> expose the host system time instead of an actual RTC.
>

virtio_rtc only registers RTC class devices for virtio_rtc clock type UTC,
so adding an UTC_SMEARED clock type would avoid leap seconds for the RTC
class.

But I understand that there are more concerns and I will re-consider. From
my POV CLOCK_{REALTIME,MONOTONIC}_ALARM support is very important however.

So the only alternative to me seems to be adding an alternative alarmtimer
backend (and time synchronization through user space).

Thanks for the comment,

Peter

2024-03-13 14:51:18

by Alexandre Belloni

[permalink] [raw]

Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

On 13/03/2024 14:06:42+0000, David Woodhouse wrote:
> If you're asking why patch 7/7 in Peter's series exists to expose the
> virtio clock through RTC, and you're not particularly interested in the
> first six, I suppose that's a fair question. As is the question of "why
> is it called virtio_rtc not virtio_ptp?".
>

Exactly my question, thanks :)

> But let me turn it around: if the kernel has access to this virtio
> device and *not* any other RTC, why *wouldn't* the kernel use the time
> from it? The fact that it can optionally *also* provide paired readings
> with the CPU counter doesn't actually *hurt* for the RTC use case, does
> it?

As long as it doesn't behave differently from the other RTC, I'm fine
with this. This is important because I don't want to carry any special
infrastructure for this driver or to have to special case this driver
later on because it is incompatible with some evolution of the
subsystem.

--
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

2024-03-13 15:36:11

by David Woodhouse

[permalink] [raw]

Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

On Wed, 2024-03-13 at 12:18 +0100, Alexandre Belloni wrote:
>
> I still don't know anything about virtio but under Linux, an RTC is
> always UTC (or localtime when dual booting but let's not care) and never
> accounts for leap seconds. Having an RTC and RTC driver behaving
> differently would be super inconvenient. Why don't you leave this to
> userspace?

Well yes, we don't need to expose *anything* from the hypervisor and we
can leave it all to guest userspace. We can run NTP on every single one
of *hundreds* of guests, leaving them all to duplicate the work of
calibrating the *same* underlying oscillator.

I thought we were trying to avoid that, by having the hypervisor tell
them what the time was. If we're going to do that, we need it to be
sufficiently precise (and some clients want to *know* the precision),
and above all we need it to be *unambiguous*.

If the hypervisor says that the time is 3692217600.001, then the guest
doesn't actually know *which* 3692217600.001 it is, and thus it still
doesn't know the time to an accuracy better than 1 second.

And if we start allowing the hypervisor to smear clocks in some other
underspecified ways, then we end up with errors of up to 1 second in
the clock for long periods of time *around* the leap second.

We need to avoid that ambiguity.

> I guess I'm still questioning whether this is the correct interface to
> expose the host system time instead of an actual RTC.

If an RTC device is able to report '23:59:60' as the time of day, I
suppose that *could* resolve the ambiguity. But talking to a device is
slow; we want guests to be able to know the time — accurately — with a
simple counter/tsc read and some arithmetic. Which means *paired* reads
of 'RTC' and the counter, and a precise indication of the counter
frequency.

Attachments:

smime.p7s (5.83 kB)

2024-03-13 18:12:40

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. The driver can read the clocks with simple
> or more accurate methods. Optionally, the driver can set an alarm.
>
> The series first fixes some bugs in the get_device_system_crosststamp()
> interpolation code, which is required for reliable virtio_rtc operation.
> Then, add the virtio_rtc implementation.
>
> For the Virtio RTC device, there is currently a proprietary implementation,
> which has been used for provisional testing.

As discussed before, I don't think it makes sense to design a new high-
precision virtual clock which only gets it right *most* of the time. We
absolutely need to address the issue of live migration.

When live migration occurs, the guest's time precision suffers in two
ways.

First, even when migrating to a supposedly identical host the precise
rate of the underlying counter (TSC, arch counter, etc.) can change
within the tolerances (e.g. ±50PPM) of the hardware. Unlike the natural
changes that NTP normally manages to track, this is a *step* change,
potentially from -50PPM to +50PPM from one host to the next.

Second, the accuracy of the counter as preserved across migration is
limited by the accuracy of each host's NTP synchronization. So there is
also a step change in the value of the counter itself.

At the moment of migration, the guest's timekeeping should be
considered invalid. Any previous cross-timestamps are meaningless, and
blindly using the previously-known relationship between the counter and
real time can lead to problems such as corruption in distributed
databases, fines for mis-timestamped transactions, and other general
unhappiness.

We obviously can't get a new timestamp from the virtio_rtc device every
time an application wants to know the time reliably. We don't even want
*system* calls for that, which is why we have it in a vDSO.

We can take the same approach to warning the guest about clock
disruption due to live migration. A shared memory region from the
virtual clock device even be mapped all the way to userspace, for those
applications which need precise and *reliable* time to check a
'disruption' marker in it, and do whatever is appropriate (e.g. abort
transactions and wait for time to resync) when it happens.

We can do better than just letting the guest know about disruption, of
course. We can put the actual counter→realtime relationship into the
memory region too. As well as being able to provide a PTP driver with
this, the applications which care about *reliable* timestamps can mmap
the page directly and use it, vDSO-style, to have accurate timestamps
even from the first cycle after migration.

When disruption is signalled, the guest needs to throw away any
*additional* refinement that it's done with NTP/PTP/PPS/etc. and revert
to knowing nothing more than what the hypervisor advertises here.

Here's a first attempt at defining such a memory structure. For now
I've done it as a "vmclock" ACPI device based loosely on vmgenid, but I
think it makes most sense for this to be part of the virtio_rtc spec.
Ultimately it doesn't matter *how* the guest finds the memory region.

Very preliminary QEMU hacks at
https://git.infradead.org/users/dwmw2/qemu.git/shortlog/refs/heads/vmclock
(I still need to do the KVM side helper for actually filling in the
host clock information, converted to the *guest* TSC)

This is the guest side. H aving heckled your assumption that we can use
the virt counter on Arm, I concede I'm doing the same thing for now.
The structure itself is at the end, or see
https://git.infradead.org/?p=users/dwmw2/linux.git;a=blob;f=include/uapi/linux/vmclock.h;hb=vmclock

From 9e1c3b823d497efa4e0acb21b226a72e4d6e8a53 Mon Sep 17 00:00:00 2001
From: David Woodhouse <[email protected]>
Date: Mon, 10 Jun 2024 15:10:11 +0100
Subject: [PATCH] ptp: Add vDSO-style vmclock support

The vmclock "device" provides a shared memory region with precision clock
information. By using shared memory, it is safe across Live Migration.

Signed-off-by: David Woodhouse <[email protected]>
---
drivers/ptp/Kconfig | 13 ++
drivers/ptp/Makefile | 1 +
drivers/ptp/ptp_vmclock.c | 404 +++++++++++++++++++++++++++++++++++
include/uapi/linux/vmclock.h | 102 +++++++++
4 files changed, 520 insertions(+)
create mode 100644 drivers/ptp/ptp_vmclock.c
create mode 100644 include/uapi/linux/vmclock.h

diff --git a/drivers/ptp/Kconfig b/drivers/ptp/Kconfig
index 604541dcb320..ace6d58c1781 100644
--- a/drivers/ptp/Kconfig
+++ b/drivers/ptp/Kconfig
@@ -131,6 +131,19 @@ config PTP_1588_CLOCK_KVM
To compile this driver as a module, choose M here: the module
will be called ptp_kvm.

+config PTP_1588_CLOCK_VMCLOCK
+ tristate "Virtual machine PTP clock"
+ depends on X86_TSC || ARM_ARCH_TIMER
+ depends on PTP_1588_CLOCK && ACPI
+ default y
+ help
+ This driver adds support for using a virtual precision clock
+ advertised by the hypervisor. This clock is only useful in virtual
+ machines where such a device is present.
+
+ To compile this driver as a module, choose M here: the module
+ will be called ptp_vmclock.
+
config PTP_1588_CLOCK_IDT82P33
tristate "IDT 82P33xxx PTP clock"
depends on PTP_1588_CLOCK && I2C
diff --git a/drivers/ptp/Makefile b/drivers/ptp/Makefile
index 68bf02078053..01b5cd91eb61 100644
--- a/drivers/ptp/Makefile
+++ b/drivers/ptp/Makefile
@@ -11,6 +11,7 @@ obj-$(CONFIG_PTP_1588_CLOCK_DTE) += ptp_dte.o
obj-$(CONFIG_PTP_1588_CLOCK_INES) += ptp_ines.o
obj-$(CONFIG_PTP_1588_CLOCK_PCH) += ptp_pch.o
obj-$(CONFIG_PTP_1588_CLOCK_KVM) += ptp_kvm.o
+obj-$(CONFIG_PTP_1588_CLOCK_VMCLOCK) += ptp_vmclock.o
obj-$(CONFIG_PTP_1588_CLOCK_QORIQ) += ptp-qoriq.o
ptp-qoriq-y += ptp_qoriq.o
ptp-qoriq-$(CONFIG_DEBUG_FS) += ptp_qoriq_debugfs.o
diff --git a/drivers/ptp/ptp_vmclock.c b/drivers/ptp/ptp_vmclock.c
new file mode 100644
index 000000000000..3c4e027090c5
--- /dev/null
+++ b/drivers/ptp/ptp_vmclock.c
@@ -0,0 +1,404 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Virtual PTP 1588 clock for use with LM-safe VMclock device.
+ *
+ * Copyright © 2024 Amazon.com, Inc. or its affiliates.
+ */
+
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/ptp_kvm.h>
+#include <linux/platform_device.h>
+#include <linux/miscdevice.h>
+#include <linux/acpi.h>
+#include <uapi/linux/vmclock.h>
+
+#include <linux/ptp_clock_kernel.h>
+
+static DEFINE_IDA(vmclock_ida);
+
+ACPI_MODULE_NAME("vmclock");
+
+struct vmclock_state {
+ phys_addr_t phys_addr;
+ struct vmclock_abi *clk;
+ struct miscdevice miscdev;
+ struct ptp_clock_info ptp_clock_info;
+ struct ptp_clock *ptp_clock;
+ enum clocksource_ids cs_id;
+ int index;
+ char *name;
+};
+
+static int vmclock_get_crosststamp(struct vmclock_state *st,
+ struct system_counterval_t *system_counter,
+ struct timespec64 *tspec)
+{
+ uint64_t cycle, delta, seq, delta_s32, delta_s64, delta_s32b;
+ uint64_t delta_lo, delta_hi, period_lo, period_hi, frac_sec_lo, frac_sec_hi;
+ int ret = 0;
+
+#ifdef CONFIG_X86
+ if (check_tsc_unstable())
+ return -EINVAL;
+#endif
+
+ preempt_disable_notrace();
+
+ do {
+
+ seq = st->clk->seq_count & ~1ULL;
+ virt_rmb();
+
+ if (st->clk->clock_status == VMCLOCK_STATUS_UNRELIABLE) {
+ ret = -EINVAL;
+ virt_rmb();
+ continue;
+ }
+
+ cycle = get_cycles();
+
+ tspec->tv_sec = st->clk->utc_time_sec;
+ frac_sec_lo = st->clk->utc_time_frac_sec & 0xffffffff;
+ frac_sec_hi = st->clk->utc_time_frac_sec >> 32;
+
+ delta = cycle - st->clk->counter_value;
+
+ delta_lo = delta & 0xffffffff;
+ delta_hi = delta >> 32;
+ period_lo = st->clk->counter_period_frac_sec & 0xffffffff;
+ period_hi = st->clk->counter_period_frac_sec >> 32;
+
+ /* Delta in units of (second >> 32) */
+ delta_s32 = delta_lo * period_hi;
+
+ /* Avoid overflow by skimming off the full seconds */
+ tspec->tv_sec += delta_s32 >> 32;
+ delta_s32 &= 0xffffffff;
+
+ if (delta_hi) {
+ tspec->tv_sec += delta_hi * period_hi;
+ delta_s32b = delta_hi * period_lo;
+
+ tspec->tv_sec += delta_s32b >> 32;
+ delta_s32 += delta_s32b & 0xffffffff;
+ }
+
+ delta_s32 += frac_sec_hi;
+
+ delta_s64 = (period_lo * delta_lo) + frac_sec_lo;
+ delta_s32 += delta_s64 >> 32;
+ delta_s64 &= 0xffffffff;
+
+ tspec->tv_sec += delta_s32 >> 32;
+ delta_s32 &= 0xffffffff;
+
+ tspec->tv_nsec = ((delta_s32 * NSEC_PER_SEC) +
+ ((delta_s64 * NSEC_PER_SEC) >> 32)) >> 32;
+
+ ret = 0;
+
+ virt_rmb();
+ } while (seq != st->clk->seq_count);
+
+ preempt_enable_notrace();
+
+ if (ret)
+ return ret;
+
+ if (system_counter) {
+ system_counter->cycles = cycle;
+ system_counter->cs_id = st->cs_id;
+ }
+
+ return 0;
+}
+
+static int ptp_vmclock_get_time_fn(ktime_t *device_time,
+ struct system_counterval_t *system_counter,
+ void *ctx)
+{
+ struct vmclock_state *st = ctx;
+ struct timespec64 tspec;
+ int ret;
+
+ ret = vmclock_get_crosststamp(st, system_counter, &tspec);
+ if (!ret)
+ *device_time = timespec64_to_ktime(tspec);
+
+ return ret;
+}
+
+
+static int ptp_vmclock_getcrosststamp(struct ptp_clock_info *ptp,
+ struct system_device_crosststamp *xtstamp)
+{
+ struct vmclock_state *st = container_of(ptp, struct vmclock_state,
+ ptp_clock_info);
+
+ return get_device_system_crosststamp(ptp_vmclock_get_time_fn, st,
+ NULL, xtstamp);
+}
+
+/*
+ * PTP clock operations
+ */
+
+static int ptp_vmclock_adjfine(struct ptp_clock_info *ptp, long delta)
+{
+ return -EOPNOTSUPP;
+}
+
+static int ptp_vmclock_adjtime(struct ptp_clock_info *ptp, s64 delta)
+{
+ return -EOPNOTSUPP;
+}
+
+static int ptp_vmclock_settime(struct ptp_clock_info *ptp,
+ const struct timespec64 *ts)
+{
+ return -EOPNOTSUPP;
+}
+
+static int ptp_vmclock_gettime(struct ptp_clock_info *ptp, struct timespec64 *ts)
+{
+ struct vmclock_state *st = container_of(ptp, struct vmclock_state,
+ ptp_clock_info);
+
+ return vmclock_get_crosststamp(st, NULL, ts);
+}
+
+static int ptp_vmclock_enable(struct ptp_clock_info *ptp,
+ struct ptp_clock_request *rq, int on)
+{
+ return -EOPNOTSUPP;
+}
+
+static const struct ptp_clock_info ptp_vmclock_info = {
+ .owner = THIS_MODULE,
+ .max_adj = 0,
+ .n_ext_ts = 0,
+ .n_pins = 0,
+ .pps = 0,
+ .adjfine = ptp_vmclock_adjfine,
+ .adjtime = ptp_vmclock_adjtime,
+ .gettime64 = ptp_vmclock_gettime,
+ .settime64 = ptp_vmclock_settime,
+ .enable = ptp_vmclock_enable,
+ .getcrosststamp = ptp_vmclock_getcrosststamp,
+};
+
+static int vmclock_miscdev_mmap(struct file *fp, struct vm_area_struct *vma)
+{
+ struct vmclock_state *st = container_of(fp->private_data,
+ struct vmclock_state, miscdev);
+
+ if ((vma->vm_flags & (VM_READ|VM_WRITE)) != VM_READ)
+ return -EROFS;
+
+ if (vma->vm_end - vma->vm_start != PAGE_SIZE || vma->vm_pgoff)
+ return -EINVAL;
+
+ if (io_remap_pfn_range(vma, vma->vm_start,
+ st->phys_addr >> PAGE_SHIFT, PAGE_SIZE,
+ vma->vm_page_prot))
+ return -EAGAIN;
+
+ return 0;
+}
+
+static ssize_t vmclock_miscdev_read(struct file *fp, char __user *buf,
+ size_t count, loff_t *ppos)
+{
+ struct vmclock_state *st = container_of(fp->private_data,
+ struct vmclock_state, miscdev);
+ size_t max_count;
+ int32_t seq;
+
+ if (*ppos >= PAGE_SIZE)
+ return 0;
+
+ max_count = PAGE_SIZE - *ppos;
+ if (count > max_count)
+ count = max_count;
+
+ do {
+ seq = st->clk->seq_count & ~1ULL;
+ virt_rmb();
+
+ if (copy_to_user(buf, ((char *)st->clk) + *ppos, count))
+ return -EFAULT;
+
+ virt_rmb();
+ } while (seq != st->clk->seq_count);
+
+ *ppos += count;
+ return count;
+}
+
+static const struct file_operations vmclock_miscdev_fops = {
+ .mmap = vmclock_miscdev_mmap,
+ .read = vmclock_miscdev_read,
+};
+
+/* module operations */
+
+static void vmclock_remove(struct platform_device *pdev)
+{
+ struct device *dev = &pdev->dev;
+ struct vmclock_state *st = dev_get_drvdata(dev);
+
+ if (st->ptp_clock)
+ ptp_clock_unregister(st->ptp_clock);
+
+ if (st->miscdev.minor == MISC_DYNAMIC_MINOR)
+ misc_deregister(&st->miscdev);
+}
+
+static int vmclock_probe_acpi(struct device *dev, struct vmclock_state *st)
+{
+ struct acpi_buffer parsed = { ACPI_ALLOCATE_BUFFER };
+ struct acpi_device *adev = ACPI_COMPANION(dev);
+ union acpi_object *obj;
+ acpi_status status;
+
+ status = acpi_evaluate_object(adev->handle, "ADDR", NULL, &parsed);
+ if (ACPI_FAILURE(status)) {
+ ACPI_EXCEPTION((AE_INFO, status, "Evaluating ADDR"));
+ return -ENODEV;
+ }
+ obj = parsed.pointer;
+ if (!obj || obj->type != ACPI_TYPE_PACKAGE || obj->package.count != 2 ||
+ obj->package.elements[0].type != ACPI_TYPE_INTEGER ||
+ obj->package.elements[1].type != ACPI_TYPE_INTEGER)
+ return -EINVAL;
+
+ st->phys_addr = (obj->package.elements[0].integer.value << 0) |
+ (obj->package.elements[1].integer.value << 32);
+
+ return 0;
+}
+
+static void vmclock_put_idx(void *data)
+{
+ struct vmclock_state *st = data;
+
+ ida_free(&vmclock_ida, st->index);
+}
+
+static int vmclock_probe(struct platform_device *pdev)
+{
+ struct device *dev = &pdev->dev;
+ struct vmclock_state *st;
+ int ret;
+
+ st = devm_kzalloc(dev, sizeof (*st), GFP_KERNEL);
+ if (!st)
+ return -ENOMEM;
+
+ if (has_acpi_companion(dev))
+ ret = vmclock_probe_acpi(dev, st);
+ else
+ ret = -EINVAL; /* Only ACPI for now */
+
+ if (ret) {
+ dev_info(dev, "Failed to obtain physical address: %d\n", ret);
+ goto out;
+ }
+
+ st->clk = devm_memremap(dev, st->phys_addr, sizeof(*st->clk),
+ MEMREMAP_WB);
+ if (IS_ERR(st->clk)) {
+ ret = PTR_ERR(st->clk);
+ dev_info(dev, "failed to map shared memory\n");
+ st->clk = NULL;
+ goto out;
+ }
+
+ if (st->clk->magic != VMCLOCK_MAGIC ||
+ st->clk->size < sizeof(*st->clk) ||
+ st->clk->version != 1) {
+ dev_info(dev, "vmclock magic fields invalid\n");
+ ret = -EINVAL;
+ goto out;
+ }
+
+ if (IS_ENABLED(CONFIG_ARM64) &&
+ st->clk->counter_id == VMCLOCK_COUNTER_ARM_VCNT) {
+ /* Can we check it's the virtual counter? */
+ st->cs_id = CSID_ARM_ARCH_COUNTER;
+ } else if (IS_ENABLED(CONFIG_X86) &&
+ st->clk->counter_id == VMCLOCK_COUNTER_X86_TSC) {
+ st->cs_id = CSID_X86_TSC;
+ }
+
+ ret = ida_alloc(&vmclock_ida, GFP_KERNEL);
+ if (ret < 0)
+ goto out;
+
+ st->index = ret;
+ ret = devm_add_action_or_reset(&pdev->dev, vmclock_put_idx, st);
+ if (ret)
+ goto out;
+
+ st->name = devm_kasprintf(&pdev->dev, GFP_KERNEL, "vmclock%d", st->index);
+ if (!st->name) {
+ ret = -ENOMEM;
+ goto out;
+ }
+
+ /* If the structure is big enough, it can be mapped to userspace */
+ if (st->clk->size >= PAGE_SIZE) {
+ st->miscdev.minor = MISC_DYNAMIC_MINOR;
+ st->miscdev.fops = &vmclock_miscdev_fops;
+ st->miscdev.name = st->name;
+
+ ret = misc_register(&st->miscdev);
+ if (ret)
+ goto out;
+ }
+
+ /* If there is valid clock information, register a PTP clock */
+ if (st->cs_id) {
+ st->ptp_clock_info = ptp_vmclock_info;
+ strncpy(st->ptp_clock_info.name, st->name, sizeof(st->ptp_clock_info.name));
+ st->ptp_clock = ptp_clock_register(&st->ptp_clock_info, dev);
+
+ if (IS_ERR(st->ptp_clock)) {
+ ret = PTR_ERR(st->ptp_clock);
+ st->ptp_clock = NULL;
+ vmclock_remove(pdev);
+ goto out;
+ }
+ }
+
+ dev_set_drvdata(dev, st);
+
+ out:
+ return ret;
+}
+
+static const struct acpi_device_id vmclock_acpi_ids[] = {
+ { "VMCLOCK", 0 },
+ {}
+};
+MODULE_DEVICE_TABLE(acpi, vmclock_acpi_ids);
+
+static struct platform_driver vmclock_platform_driver = {
+ .probe = vmclock_probe,
+ .remove_new = vmclock_remove,
+ .driver = {
+ .name = "vmclock",
+ .acpi_match_table = vmclock_acpi_ids,
+ },
+};
+
+module_platform_driver(vmclock_platform_driver)
+
+MODULE_AUTHOR("David Woodhouse <[email protected]>");
+MODULE_DESCRIPTION("PTP clock using VMCLOCK");
+MODULE_LICENSE("GPL v2");
diff --git a/include/uapi/linux/vmclock.h b/include/uapi/linux/vmclock.h
new file mode 100644
index 000000000000..cac24536c5c8
--- /dev/null
+++ b/include/uapi/linux/vmclock.h
@@ -0,0 +1,102 @@
+/* SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause) */
+
+/*
+ * This structure provides a vDSO-style clock to VM guests, exposing the
+ * relationship (or lack thereof) between the CPU clock (TSC, timebase, arch
+ * counter, etc.) and real time. It is designed to address the problem of
+ * live migration, which other clock enlightenments do not.
+ *
+ * When a guest is live migrated, this affects the clock in two ways.
+ *
+ * First, even between identical hosts the actual frequency of the underlying
+ * counter will change within the tolerances of its specification (typically
+ * ±50PPM, or 4 seconds a day). The frequency also varies over time on the
+ * same host, but can be tracked by NTP as it generally varies slowly. With
+ * live migration there is a step change in the frequency, with no warning.
+ *
+ * Second, there may be a step change in the value of the counter itself, as
+ * its accuracy is limited by the precision of the NTP synchronization on the
+ * source and destination hosts.
+ *
+ * So any calibration (NTP, PTP, etc.) which the guest has done on the source
+ * host before migration is invalid, and needs to be redone on the new host.
+ *
+ * In its most basic mode, this structure provides only an indication to the
+ * guest that live migration has occurred. This allows the guest to know that
+ * its clock is invalid and take remedial action. For applications that need
+ * reliable accurate timestamps (e.g. distributed databases), the structure
+ * can be mapped all the way to userspace. This allows the application to see
+ * directly for itself that the clock is disrupted and take appropriate
+ * action, even when using a vDSO-style method to get the time instead of a
+ * system call.
+ *
+ * In its more advanced mode. this structure can also be used to expose the
+ * precise relationship of the CPU counter to real time, as calibrated by the
+ * host. This means that userspace applications can have accurate time
+ * immediately after live migration, rather than having to pause operations
+ * and wait for NTP to recover. This mode does, of course, rely on the
+ * counter being reliable and consistent across CPUs.
+ */
+
+#ifdef __KERNEL__
+#include <linux/types.h>
+#else
+#include <stdint.h>
+#endif
+
+struct vmclock_abi {
+ uint32_t magic;
+#define VMCLOCK_MAGIC 0x4b4c4356 /* "VCLK" */
+ uint16_t size; /* Size of page containing this structure */
+ uint16_t version; /* 1 */
+
+ /* Sequence lock. Low bit means an update is in progress. */
+ uint64_t seq_count;
+
+ /*
+ * This field changes to another non-repeating value when the CPU
+ * counter is disrupted, for example on live migration.
+ */
+ uint64_t disruption_marker;
+
+ uint8_t clock_status;
+#define VMCLOCK_STATUS_UNKNOWN 0
+#define VMCLOCK_STATUS_INITIALIZING 1
+#define VMCLOCK_STATUS_SYNCHRONIZED 2
+#define VMCLOCK_STATUS_FREERUNNING 3
+#define VMCLOCK_STATUS_UNRELIABLE 4
+
+ uint8_t counter_id;
+#define VMCLOCK_COUNTER_INVALID 0
+#define VMCLOCK_COUNTER_X86_TSC 1
+#define VMCLOCK_COUNTER_ARM_VCNT 2
+
+ uint8_t pad[3];
+
+ /*
+ * UTC on its own is non-monotonic and ambiguous.
+ *
+ * Inform the guest about the TAI offset, so that it can have an
+ * actual monotonic and reliable time reference if it needs it.
+ *
+ * Also indicate a nearby leap second, if one exists. Unlike in
+ * NTP, this may indicate a leap second in the past in order to
+ * indicate that it *has* been taken into account.
+ */
+ int8_t leapsecond_direction;
+ int16_t tai_offset_sec;
+ uint64_t leapsecond_counter_value;
+ uint64_t leapsecond_tai_time;
+
+ /* Paired values of counter and UTC at a given point in time. */
+ uint64_t counter_value;
+ uint64_t utc_time_sec;
+ uint64_t utc_time_frac_sec;
+
+ /* Counter frequency, and error margin. Units of (second >> 64) */
+ uint64_t counter_period_frac_sec;
+ uint64_t counter_period_error_rate_frac_sec;
+
+ /* Error margin of UTC reading above (± picoseconds) */
+ uint64_t utc_time_maxerror_picosec;
+};
--
2.44.0

Attachments:

smime.p7s (5.83 kB)