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

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Message-ID: <03d4652d-5bc5-439e-ba32-b17170709584@opensynergy.com>
Date: Fri, 28 Jun 2024 13:33:22 +0200
From: Peter Hilber <peter.hilber@...nsynergy.com>
To: David Woodhouse <dwmw2@...radead.org>, 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 27.06.24 18:03, David Woodhouse wrote:
> 
> I've updated the tree at
> https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/vmclock
> (but not yet the qemu one).
> 
> I think I've taken into account all your comments apart from the one
> about non-64-bit counters wrapping. I reduced the seq_count to 32 bit
> to make room for a 32-bit flags field, added the time type
> (UTC/TAI/MONOTONIC) and a smearing hint, with some straw man
> definitions for smearing algorithms for which I could actually find
> definitions.
> 
> The structure now looks like this:
> 
> 
> struct vmclock_abi {

[...]

> 
> 	/*
> 	 * What time is exposed in the time_sec/time_frac_sec fields?
> 	 */
> 	uint8_t time_type;
> #define VMCLOCK_TIME_UNKNOWN		0	/* Invalid / no time exposed */
> #define VMCLOCK_TIME_UTC		1	/* Since 1970-01-01 00:00:00z */
> #define VMCLOCK_TIME_TAI		2	/* Since 1970-01-01 00:00:00z */
> #define VMCLOCK_TIME_MONOTONIC		3	/* Since undefined epoch */
> 
> 	/* Bit shift for counter_period_frac_sec and its error rate */
> 	uint8_t counter_period_shift;
> 
> 	/*
> 	 * Unlike in NTP, this can indicate a leap second in the past. This
> 	 * is needed to allow guests to derive an imprecise clock with
> 	 * smeared leap seconds for themselves, as some modes of smearing
> 	 * need the adjustments to continue even after the moment at which
> 	 * the leap second should have occurred.
> 	 */
> 	int8_t leapsecond_direction;
> 	uint64_t leapsecond_tai_sec; /* Since 1970-01-01 00:00:00z */
> 
> 	/*
> 	 * Paired values of counter and UTC at a given point in time.
> 	 */
> 	uint64_t counter_value;
> 	uint64_t time_sec; /* Since 1970-01-01 00:00:00z */

Nitpick: The comment is not valid any more for TIME_MONOTONIC.