linux-kernel - Re: [RFC PATCH v2] ptp: Add vDSO-style vmclock support

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Message-ID: <24a6fd46a3383aa8d18d19511b8422baad37317c.camel@infradead.org>
Date: Fri, 05 Jul 2024 16:02:46 +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>, "Chashper, David" <chashper@...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 Fri, 2024-07-05 at 10:12 +0200, Peter Hilber wrote:
> On 03.07.24 12:40, David Woodhouse wrote:
> 
> [...]
> 
> > 
> > 
> > This is what I currently have for 'struct vmclock_abi' that I'd like to
> > persuade you to adopt. I need to tweak it some more, for at least the
> > following reasons, as well as any more you can see:
> > 
> >  • size isn't big enough for 64KiB pages
> >  • Should be explicitly little-endian
> >  • Does it need esterror as well as maxerror?
> 
> I have no opinion about this. I can drop esterror if unwanted.

I also don't care. I'm just observing the inconsistency.

> >  • Why is maxerror in picoseconds? It's the only use of that unit

Between us we now have picoseconds, nanoseconds, (seconds >> 64) and
(seconds >> 64+n).

The power-of-two fractions seem to make a lot of sense for the counter
period, because they mean we don't have to perform divisions.

Does it makes sense to harmonise on (seconds >> 64) for all of the
fractional seconds? Again I don't have a strong opinion; I only want us
to have a *reason* for any differences that exist.

> >  • Where do the clock_status values come from? Do they make sense?
> >  • Are signed integers OK? (I think so!).
> 
> Signed integers would need to be introduced to Virtio, which so far only
> uses explicitly unsigned types: u8, le16 etc.

Perhaps. Although it would also be possible (if not ideal) to define
that e.g. the tai_offset field is a 16-bit "unsigned" integer according
to virtio, but to be interpreted as follows:

If the number is <= 32767 then the TAI offset is that value, but if the
number is >= 32768 then the TAI offset is that value minus 65536.

Perhaps not pretty, but there isn't a *fundamental* dependency on
virtio supporting signed integers as a primary type.

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