linux-kernel - Re: [RFC PATCH v3] ptp: Add vDSO-style vmclock support

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Message-ID: <20240708091708.GJ1481495@kernel.org>
Date: Mon, 8 Jul 2024 10:17:08 +0100
From: Simon Horman <horms@...nel.org>
To: David Woodhouse <dwmw2@...radead.org>
Cc: 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>,
	"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>,
	qemu-devel <qemu-devel@...gnu.org>
Subject: Re: [RFC PATCH v3] ptp: Add vDSO-style vmclock support

On Sat, Jul 06, 2024 at 04:14:39PM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> The vmclock "device" provides a shared memory region with precision clock
> information. By using shared memory, it is safe across Live Migration.
> 
> Like the KVM PTP clock, this can convert TSC-based cross timestamps into
> KVM clock values. Unlike the KVM PTP clock, it does so only when such is
> actually helpful.
> 
> The memory region of the device is also exposed to userspace so it can be
> read or memory mapped by application which need reliable notification of
> clock disruptions.
> 
> The shared memory structure is intended to be adopted into the nascent
> virtio-rtc specification (since one might consider a virtio-rtc
> specification that doesn't fix the live migration problem to be not fit
> for purpose). It can also be presented via a simple ACPI device.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>

...

> diff --git a/drivers/ptp/ptp_vmclock.c b/drivers/ptp/ptp_vmclock.c

...

> +	/* 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));

Hi David,

As per my comment on v2, although it is harmless in this case,
it would be nicer to use strscpy() here and avoid fortification warnings.


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